The 2-limit consists of intervals that are either an integer whose only prime factor is 2, or the reciprocal of such an integer. Naturally, since 2/1 is the octave, this limits us to unisons, octaves and stacks of octaves. The 2-limit is fundamental to any sort of edo, with 1edo being the easiest and simplest to grasp.
Due to the human ability to perceive notes an octave apart as having the same pitch class, the 2-limit also collapses very easily, with this collapse being generally implemented in lattices. This collapse is helpful to understanding the pitches involved in other prime axes within the space of a single octave, should you add other primes to the mix.
The 2-limit is equivalent to the 1-odd-limit.