A regular temperament is a kind of abstract musical system that looks the same no matter which pitch you start from (or consider the "tonic"). In other words, unlimited free modulation is possible - any interval can be stacked as many times as you like. Regular temperaments generally have an infinite number of notes; and other than equal temperaments, every regular temperament actually has an infinite number of notes in between any two other notes!
In addition to unlimited modulation, regular temperaments are usually thought of as being "tempered versions" of some more complicated system of "pure" or "target" intervals, very often just intonation (JI). A temperament only qualifies as a regular temperament if this approximation works in a perfectly consistent way - for example, the sum of two tempered intervals must always be the tempered version of the sum of the JI intervals. Multiple pure intervals may be represented by the same tempered interval (so they're "tempered together"), but a single pure interval must never be represented by different tempered intervals - if so, the temperament is irregular.
One particularly simple kind of regular temperaments are the equal temperaments, which represent all intervals by multiples of a single smallest step.
At the other extreme, JI itself can be considered a kind of "temperament" where no tempering is happening (no commas are tempered out but all are preserved as small pitch differences).
In between lies the cornucopia of temperaments discussed in Paul Erlich's seminal work, A Middle Path Between Just Intonation and the Equal Temperaments.
Introductions to regular temperament theory
Why would I want to use a regular temperament?
Regular temperaments are of most use to musicians who want their music to sound as much as possible like low-overtone just intonation, but without the difficulties normally associated with low-overtone JI, such as wolf intervals, commas, and comma pumps. Specifically, if your chord progression pumps a comma, and you want to avoid pitch shifts, wolf intervals, and/or tonic drift, that comma must be tempered out. Temperaments are also of interest to musicians wishing to exploit the unique possibilities that arise when ratios that are distinct in JI become equated. For instance, 10/9 and 9/8 are equated in meantone. Equating distinct ratios through temperament allows for the construction of musical "puns", which are melodies or chord progressions that exploit the multiplicity of "meanings" of tempered intervals. Finally, some use temperaments solely for their sound. For example, one might like the sound of neutral 3rds, without caring much what ratio they are tuned to. Thus one might use Rastmic even though no commas are pumped.
What do I need to know to understand all the numbers on the pages for individual regular temperaments?
Although the concept of regular temperament is centuries old and predates much of modern mathematics, members of the Yahoo! Alternative Tuning List have developed a particular form of numerical shorthand for describing the properties of temperaments. The most important of these are vals (aka mappings) and tempering out commas, which any student of the modern regular temperament paradigm should become familiar with. These concepts are rather straightforward and require little math to understand.
The rank of a temperament equals the number of primes in the subgroup minus the number of linearly independent (i.e. non-redundant) commas that are tempered out.
Another recent contribution to the field of temperament is the concept of optimization, which can take many forms. The point of optimization is to minimize the difference between a temperament and JI by finding an optimal tuning for the generator. The two most frequently used forms of optimization are POTE ("Pure-Octave Tenney-Euclidean") and TOP ("Tenney OPtimal", or "Tempered Octaves, Please"). Optimization is rather intensive mathematically, but it is seldom left as an exercise to the reader; most temperaments are presented here in their optimal forms in terms of POTE generators. In addition, for each temperament there is a list of EDOs showing possible EDO tunings in the order of better accuracy.
Yet another recent development is the concept of a pergen, appearing here as (P8, P5/2) or somesuch. Every rank-2, rank-3, rank-4, etc. temperament has a pergen, which specifies the period and the generator(s). Assuming the prime subgroup includes both 2 and 3, a rank-2 temperament's period is either an octave or some fraction of it, and its generator is either a 5th or some fraction of some 3-limit interval. Since both period and generator are conventional musical intervals or some fractions of them, the pergen gives great insight into notating a temperament. Several temperaments may share the same pergen, in fact, every strong extension of a temperament has the same pergen as the original temperament. Thus pergens classify temperaments but don't uniquely identify them. "c" in a pergen means compound (widened by one octave), e.g. ccP5 is a 5th plus two 8ves, or 6/1.
Each temperament has two names: a traditional name and a color name. The traditional names are arbitrary, but the color names are systematic and rigorous, and the comma can be deduced from the color name. Wa = 3-limit, yo = 5-over, gu = 5-under, zo = 7-over, and ru = 7-under. See also Color Notation/Temperament Names.