A very important part of reading any sort of music is understanding what a time signature is, or another word for it could be a meter. And a time signature are the two numbers that fall on top of each other at the beginning of a line of music. You'll see it if you're ever looking at music at the beginning of a groove or a melody. You'll see two numbers that fall on top of each other.
The most common time signature is four-four. In fact, it's actually called common time. And what four-four means is that it refers to two specific numbers. The top number asks the question of how many. What I mean by how many is how many numbers are you going to count in the measure? In this case, you're talking about four. So you'd count one, two, three, four.
The bottom number, however, is a very important number that lets you know what kind of note is getting each count. So in this case since it's a four, you're talking about quarter notes. So on four-four, it basically means that you're getting four counts to a measure, and quarter notes are getting each count of time. One, two, three, four. And when you change meters, the numbers are going to do two things. The top number changing numbers will change the count that you're actually physically counting up to, and the bottom number is going to change the rate of time. Now the bottom number can only be a number that refers to your note values. So a four refers to quarter notes. An eight refers to eighth notes. A sixteen refers to sixteenth notes.
And when you're changing meters and it goes from a different meter on the bottom, the note value is what changes. So if it goes from a four to an eight, you're talking about going twice as fast, because you're going from a quarter note value to an eighth note value.
So let's say I was doing a measure of four-four, going to a measure of seven-eight. Two things would have to happen. You'd count from four to seven. And your note value would double, going from quarter notes to eighth notes on the bottom. So that would sound like one, two, three, four. One, two, three, four, five six, seven. One, two, three, four. One, two, three, four, five, six, seven.
That's just the beginning of time signature changing, but it's good to understand what those numbers mean, and that will get you started on understanding it.