The top number gives the number of beats per bar (measure). The bottom number works sort of like a fraction: if it's a 4, then the quarter note is the one that gets the beat. So 4/4 time means 4 beats to the measure, using the quarter note to count the beats. For fast music, the eighth note might be used: so in 6/8 time, it's six beats to the measure, the eighth note gets the beat. Of course, you can also count 6/8 time in sets of 2, 9/8 time in 3s, and stuff like that. But that's the gist of it.