When reading music you may come across a term called note values, and what that basically refers to is the length of time that each note is worth. And you might also hear a term called the rhythm scale, and basically what the rhythm scale is the way math works, is you start with the one and you divide into two, and eventually goes to four and eight, so on and so forth, and the standard note value rhythmic scale starts with the whole note.
Now if you're talking about a one measure worth of time, you're talking about four counts. A whole note is worth all four of those counts. So if I was going to play or count a whole note in a bar's worth of time, it would just strike on one and it would last the entire measure or the entire bar, which is the same word. It would sound something like this, one, two, ready, and, one, two three, four, two, three four. When the whole note is divided in half, you end up getting exactly that, you get two half notes. And on a full bar of time, a half note would fall on beat one and beat three, something like one, three, one three. Two, four, two, four. Now when you divide those in half, now you're talking about quarter notes. Quarter notes or four are four beats in each measure, and they fall on all the numbered counts of the measure, one, two, three, four, those are your quarter notes.
Now when you divide quarter notes they split into two notes apiece, and that turns into eighth notes, and the way you count eighth notes is you insert an and in-between each number, one and two and three and four, and one and two and three and four and . . . And finally when we cover eighth notes going to sixteenth notes, sixteenth notes double in speed, and now you're talking about going to one ee and a two ee and a three ee and a four ee and a, one ee and a is one set of sixteenth notes, and that would sound something like this.
There are more note values, such as thirty-second notes or sixty-fourth notes, but it starts to get out of a normal tempo range, so that's a good range to start with when it comes to note values.