What is the time signature of samba music?

The bottom number tells you the value of the beat. The top number tells you the number of "beats to the bar".

In 3 - 4 time there are 3 crotchet (quarter note) beats to the bar.

When the bottom number is 4 the beat is a quarter note, a crotchet. When the bottom number is 2 the beat is a half note, a minim.