The two numbers at the beginning of a song are called the

It's called the time signature

To start this off, think of a waltz. You might count it out like this: One two three one two three (and so on). That's 3/4 time; each measure is three quarter-notes long (or the equivalent number of notes of other lengths). Most music is in 4/4, also known as common time, where measures are four quarter-notes long.