it's stupidly complicated. but the basic idea is that the first two numbers (e.g. 32) is the band size, which is the size of their chest without accounting for their bust and the letter following it (e.g. 32D) accounts for the bust. What complicates things is the system of sister sizes. For example, a 32D amounts to roughly the same breast volume as a 34C or a 30DD (in theory). So someone who says "I'm a double D
" might not actually be that big, since a 28DD is equivalent to a 34B (which is average-ish).
Also worth noting that most women wear the wrong bra size, since most of the popular stores that sell bras just size people into mainstream sizes (i.e. 34s and 36s) because they don't stock the sizes that they actually need (i.e. 28s and 30s).
Hope that helps.