The pentatonic scales
Penta = five. A pentatonic scale is a five-note scale — and the trick to why it sounds so easy on the ear is that it has no half steps. No half steps means no grinding dissonances, so almost any combination of the notes sounds fine. Folk music the world over uses pentatonic scales; so do nursery tunes, hymns, and a lot of pop.
Major pentatonic
Take a major scale and drop the 4th and 7th — the two "restless" notes. What's left is ^1 ^2 ^3 ^5 ^6: open, sunny, uncomplicated.
Amazing Grace is almost entirely major pentatonic. So is Auld Lang Syne, and the black keys of a piano.
Minor pentatonic
From the natural minor, drop the 2nd and 6th. What's left is ^1 ♭^3 ^4 ^5 ♭^7 — the sound of blues and rock riffs.
Notice: G major pentatonic and E minor pentatonic are the same five notes — G A B D E — just started from a different home. The relative relationship again.
Try it
Try
Turn on a G drone. Play only the notes G, A, B, D, E — in any order, any rhythm, up and down the string. Make up a little tune. Every note works against the drone. That "everything fits" feeling is why pentatonic scales are where beginners (and improvisers) start.
The usual mistake
Thinking pentatonic scales are "incomplete" or "for beginners only." They're a deliberate choice — leaving out the two notes that create tension gives you a scale that's pure melody, no friction. Debussy, Dvořák and countless film composers use pentatonic passages on purpose, for exactly that open, weightless sound.