What does 16 bars mean in music?

That depends entirely on the time signature of the music. If it is in 4/4 time there are 64 beats. If it is in 2/2 time there are 32 beats. If it is in 3/4 time there are 48 beats.

In a time signature the first number (on top) is how many beats per measure.

The second number (on bottom) is what note gets the beat. For example, in 4/4 time there are 4 beats per measure, and a quarter note is one beat. In 2/2 time there are 2 beats per measure, and a half note is one beat. In 3/4 time there are 3 beats per measure, and a quarter note is one beat.