The cycle of 4th’s is movement based on the interval (distance) of a 4th.
On the guitar, that distance is 2 ½ steps (or 5 half steps).
When starting on the note C, the cycle progresses as follows moving through all 12 keys before starting over: C - F - Bb - Eb - Ab - Db - Gb - B - E - A - D - G
Remember about enharmonic equivalents which are notes that sound exactly the same, but spelled differently. Bb/A#, Eb/D#, Ab/G#, Db/C#, Gb/F#
This cycle is an important fundamental of Western harmonic motion. Most jazz standards and pop music contain harmony that moves with cycle 4 root motion at some point in the song. The common ii-V-I progression is cycle 4 motion.
Memorizing this cycle and playing it on the 6th and 5th string of the guitar is a great place to start.
Musicians often practice material in all 12 keys by moving through the cycle of fourths. Scales, arpeggios, chord forms, and even entire songs can be taken through the cycle.