all that does is change where the downbeat (big white tick) is. if you dont know what that means, just always stick to 4/4 tbh, its usually that anyway. if you're wrong someone will point it out in modding.
it only really matters for music theory things and nightcore/the main menu effects so until you are making good maps you want to push its probably the least important part of timing so dont worry too much.
its the time signature, it describes the measures of the song, top number is number of beats in the measure, bottom number is what a beat is. 4/4 means there are 4 quarter notes in a measure - by far the most common - its often just called common time instead of 4/4, 3/4 means there are 3 quarter notes in a measure osu calls it waltz because waltz music is in 3/4 but not all 3/4 music is a waltz, 5/4 is 5 beats per measure, etc.
osu doesnt let you change the denominator, but for example 6/8 means there are six eighth notes in one measure. mapping these time signatures requires some workarounds to get the metronome to line up to the emphasized beats - like mapping with 1/3 beatsnap and scaling bpm to fit in a 2/4 time signature.