On a piano a scale is a simple row of keys. On a guitar the same notes appear in many places on the neck, so we need a way to organise them. Guitarists do that with scale positions, or just positions.
A position is one place on the neck where your hand stays while you play the scale across all six strings. Each finger looks after one fret, so a position covers about four frets, sometimes with a small stretch to reach one more.
Guitarists learn five basic near the top of the neck: C, A, G, E and D. Each has its own shape under the fingers, and you can slide a shape up the neck to play other chords. The CAGED system builds one scale position around each of these five shapes, so you always have a chord shape to hold on to. (You'll learn the chords themselves in Chords on the Guitar.)
Here is G major's E-shape position: the E chord shape, slid up the neck so it plays G. The filled notes are the root, G; the teal rings show the chord shape, here a G major chord, inside the scale; the numbers are your fingers (1 = index … 4 = little finger).
The five positions together cover the whole neck, and neighbouring positions share notes, so you can slide from one into the next.
Another system, three notes per string, plays exactly three notes on every string. It gives seven positions, one starting on each note of the scale, with a very regular finger pattern that makes fast runs easier. The positions are a little wider, so they need more stretching. Here is G major's first position, starting on the root:
Many players learn CAGED first, to find their way around the neck, and three notes per string later, for speed.
Pentatonic scales have only five notes, so they're usually played in five boxes, with two notes on every string. Here is the first box of A minor pentatonic:
The fretboard tool shows every position of the major, minor and pentatonic scales, on any root note. It also has a Theory view, which shows where the notes are, next to the Fingering view, which shows how to play them.
Next: Modes