The Circle of Fifths is a graphic representation of 2 different harmonic ideas.
The first is that key centers that are a perfect 5th apart also are only different by one sharp or flat. G major has 1 sharp, D major has 2 sharps. Each key center and the key on either side of it on the Circle of Fifths are called Closely Related Keys (CRKs). You’ll also see that each key signature has 2 keys—a major one and a minor one—that share the same key signature. These are called relative major and minor keys.
The second idea that the Circle of Fifths shows is a relationship of chords that can occur a fifth apart. If you ignore the key signatures and think of each letter as a chord, you can create an endless cycle for a chord progression, called a Circle of Fifths progression. Or, if you want to stay in a single key, the key center and the chords on either side of it when you click on it will all sound good together.