The name says it all: in the circle of fifths every key is exactly one fifth away from each of its two neighbours. One step clockwise always means a fifth higher. This way, closely related keys – those with almost identical key signatures – always sit next to each other in the circle, while distant keys sit opposite one another.
Going anticlockwise you correspondingly get a circle of fourths: from C to F (anticlockwise), for example, is a perfect fourth.
The inner ring shows, for each major key, its relative minor key – the minor key with the same key signature, i.e. the same notes in the scale (e.g. C major and A minor).
The outer ring shows the function of each key relative to the tonic you click: the dominant is the fifth above the root (clockwise, the right-hand neighbour), the subdominant is the fourth (anticlockwise, the left-hand neighbour). Both are closely related to the tonic, which is why many pieces of music move between them and their respective minor keys.
New to scales? To understand the circle of fifths we recommend first learning what a scale actually is and how it is built. We explain that step by step here. Scales explained →
Click a key to make it the tonic – the circle rotates it to the top and the notes panel shows the scale and chords of that key.