The Circle of 5ths is exactly that; a circle with the twelve tones of the Chromatic Scale spaced evenly around the perimeter one fifth interval apart. You can think of it the same way the twelve hours are spaced around a clock-face five minutes apart. What’s the reason for The Circle? You can use it as a tool to understand the organic nature of how chords and chord progressions are organized in western music.
Pick any note on the circle to start on. This will be the key you will be playing in. Let’s choose C at the 12 noon position on the circle. The fifth of C is G, the note immediately to the right of C in the 1 o’clock position. Count it 1-C 2-D 3-E 4-F 5-G. G is the fifth note of the C Major Scale.
If we make the fifth, in this case G, a 7th chord it will introduce a tension in the G Major chord that will make the G7 want to resolve back to the root C. We can substitute Roman numerals for the root (I) and the fifth (V) or the dominant 7th chord (V7). Use this method to transpose to any key on the circle and play two-chord songs in any key.