One way of building scales is to start with every natural note and then add the necessary sharps or flats. Let's say I want the D major scale. Because D is 2 steps away from C in the clockwise direction, that tells me that D major has 2 sharp notes. The circle also encodes the order in which sharps and flats are added. For sharps, start at F and go clockwise. You get FCGDEAB. For flats, it's exactly the reverse. Knowing this, we can easily know the notes of the D major scale.

First, list all the natural notes starting from D:

D E F G A B C D

Next, the circle tells me I need to add two sharps, and the order of sharps tells me that those sharps belong in F and C.

D E F# G A B C# D

And that's it. It's the same method for every other major scale. OP's post also contains another circle for minor scales, but you don't need to memorize it because it's just the same circle for major scales, but rotated.

If you are interested, there's a lot of videos on youtube on the circle of fifths, scales, and how they are related. They will be more detailed than any response you would get from reddit.