Seven scale degrees
In a major key, notes are numbered 1 through 7 from the tonic. Chords built on those notes inherit the same degree numbers, which are commonly written as Roman numerals for harmony.
Diatonic triad qualities
In a major key the basic triad pattern is major, minor, minor, major, major, minor, diminished. That pattern explains why I, IV, and V are major while ii, iii, and vi are minor.
Why I–IV–V is useful
I–IV–V contains three strong diatonic functions and appears in many styles. On guitar it is also a practical framework for learning how the same three harmonic roles can be voiced in several neck areas.
Roman numerals make transposition easier
If you understand a progression as I–IV–V instead of only C–F–G, you can move the idea to another key while preserving the relationships.
Use the degrees on the fretboard
Fretwise labels diatonic relationships inside the selected key and uses them across Position Path, Triad Path, Progression Trainer, Route Drill, and Ear Trainer.
Keep learning
Use these ideas as practice concepts rather than rules. The useful question is whether they make the fretboard easier to navigate and the harmony easier to hear.