Frequency
Frequency is the inverse of time. In music, specifically it refers to the number of oscillations per unit of time of a vibrating object. Frequency is typically measured in hertz (Hz), defined as one oscillation per second.
A sound's pitch is associated with the fundamental frequency of its frequency spectrum.
Definition
By convention, frequency refers to the linear measure of how "high" or "low" a sound is. That is, a sound that is twice the frequency has twice as many oscillations per second.
Usually, absolute frequency in Hz is not of interest to musicians (even in xenharmony), because human hearing perceives ratios of frequency instead of absolute frequencies. Because of this, a standard frequency is usually set (by convention, at 440 Hz, which corresponds to the A above middle C), and the frequencies of other notes in the scale are defined by multiplying that base frequency by ratios.
Frequency is different from pitch, which is the logarithmic measure of how "high" or "low" a sound is. Multiplications in frequency translate to additions in pitch; the notes in an equal tuning are equally spaced in pitch.
Overtones
Frequency ratios that take the form of simple fractions are concordant. This is because of the way notes sound when played on most instruments: the frequency of the note being heard is called the fundamental frequency (say 440 Hz), but the sound wave also contains other frequencies at integer multiples of the fundamental, called harmonics or overtones: the perfect octave at twice the frequency (880 Hz), the third harmonic at 3 times the frequency (1320 Hz), and so on. The relative volume of these harmonics determines what sound the instrument has. When the harmonics of two sound waves coincide, the two frequencies are considered concordant. This obviously happens with two sound waves that have the same frequency (i.e. a ratio of 1/1), but also happens with other simple ratios, which include harmonics, but also include ratios like 3/2 and 5/4. Because of this, concordance can be considered how much two notes sound like "the same note".