When reading the The Circle of 5ths from left to right it depicts the 12 keys according to the progressive order of sharps, and when read from right to left it depicts the keys according to the progressive order of flats.
For example the Key of C has no sharps and no flats, and the Key of C# has 7 sharps.
The Key of F has one flat and the Gb has 6 flats.
The key signatures are not listed in the diagram above so the number of sharps and flats are not necessarily obvious in this depiction. You can easily use any search engine to find other Circle of 5th diagrams that explicitly depict the key signatures of each key.
The Circle of 5ths also shows all possible cadences. A Cadence is a harmonic movement from the V7 chord in a key to the I chord in the key. A cadence produces a sound that you would recognize. You can find a cadences at the end of a lot classical music, in jazz, blues and pop. Cadences are very common and important.
If you will read the circle of 5ths from left to right, you will notice that
B is the V chord in the Key of E
E is the V chord in the Key of A
A is the V chord in the Key of D
D is the V chord in the Key of G
G is the V chord in the Key of C
The Circle of fifths is also used to produce some very common chord progressions based on V-->I movement. V-->I (Five to one) movement is also called cadential movement.
The I vii iii vi ii V7 is a Chord Progression that is based on the circle of 5ths. There are three additional cadential chord progressions enfolded within this progression:
1) ii V I
2) vi ii V I
3) iii vi ii V I
4) vii iii vi ii V I
In the Key of C, Bm7b5 is the vii, E minor is the iii, A minor is the iv, D minor is the ii chord, G7 is the dominant V chord and C is the I chord.
Notice that the circle of 5ths also shows all of the possible vii iii vi ii V I chord progressions