The perfect fifth on guitar
On guitar, the perfect fifth is two frets up and three frets up when you cross to the next string, and seven frets up along one string.
One pair of strings is the odd one out, because the gap between those two strings is different from the rest. Most players learn that exception as a rule of its own.
An interval is the distance between two notes, and on a fretted instrument that distance becomes a shape. A perfect fifth is seven semitones. You can play it as one note and another two frets higher on the next string, and you can slide that pair anywhere you like. The notes change as you move it, but the distance between them stays the same.
This is why guitarists think in shapes. A pianist playing a fifth has to know which two keys to press. You need to know one grip and where to put it. There is one exception to keep in mind. Crossing from the third string to the second, the uneven gap in the tuning means a fifth is three frets up rather than two.
Every chord and every scale in the rest of this course is built by stacking intervals, so the shapes you learn here are the raw material for all of it.