An SI unit (short for Système International) is a very thoroughly standardised and understood means of measuring something.
For the purposes of talking about music and microcomputers (two of the very few things I know anything about), the two SI units to concern ourselves with most are the base unit of seconds, and the derived unit of Hertz, the latter literally being 1 ÷ seconds.
To reduce verbosity, SI units can be prefixed to multiply or divide them by multiples of a thousand. At the scales we're dealing with here, we only need to concern ourselves with a handful of SI prefixes:
For example, if a waveform repeats a thousand times every second, we can say it has a frequency of 1 kilohertz, or 1 kHz for short, and that each cycle lasts 1 millisecond, or 1 ms for short. If it repeats a million times every second, its frequency is 1 MHz, and each cycle lasts 1 μs.
For computers in particular, their memory is almost always binary, so instead of measuring bits, nibbles, bytes, or words in multiples of 1,000 (×103), we measure them in multiples of 1,024 (×210).
This is a pretty substantial abuse of SI prefixes, from the uppercase K (which is, admittedly, neatly consistent) to the rounding off in binary.
In order to disambiguate binary prefixes from their decimal counterparts, it's now recommended to replace their last syllable with bi for binary, and add a lowercase letter i between the prefix abbreviation and unit of measurement, as in KiB for kibibyte rather than KB for kilobyte.
To my knowledge, this hasn't yet gained much traction, as historically people in the computer industry have been long accustomed to appropriating the decimal prefixes as they are.
At any rate, SI prefixes, along with SI units themselves, are wonderfully convenient.
SI units: Hertz