Strictly speaking it just means 2 - 2 makes a couple, as in bf/gf, husband/wife etc. But, sometimes it can be used more loosely, e.g ‘just give me a couple of minutes’ really means ‘wait a few minutes’. ‘Few’ is one up from ‘couple’, and means roughly 3 or 4 things (or maybe one or two more) not a definite number but more than 2.