With time signatures, we will always see 2 numbers in the form of a fraction. (Yes, music has math too!) So we'll see a number on top, and a number on the bottom. The top number represents how many of a certain note, and the bottom number represents what kind of a certain note. If you haven't already, be sure to check out this lesson to understand the very basics of time signatures.
So with our 4/4 time signature (also known as the common time signature), we know we're dealing with quarter notes since the bottom number is a 4. And since the top number means "how many", we can conclude that the 4/4 time signature represents four quarter notes in a measure.
You can count the beats of a 4/4 time signature as follows: "One Two Three Four"
In each "Overview" video, you'll notice in the video that I play quarter notes, eighth notes, and sixteenth notes along with a metronome on the Low E string. This metronome is set to play within the given time signature. As we play the different kind of rhythmic notes along with it, it will help our understanding of how the time signature works.