"Penta" means five: a pentatonic scale has five notes. It's probably the most used scale in rock, pop, blues and folk, and the easiest one to make up melodies with.
You met this scale in Why Twelve Notes?, built from five stacked 5ths (C G D A E). Here's another way to get it: take the C major scale and leave out two notes, the 4th and the 7th (F and B):
Why those two? C major has two half steps, E–F and B–C. Leaving out F and B removes both of them. Without half steps, no two notes of the scale rub against each other, so it's hard to play a note that sounds wrong. That makes the pentatonic scale perfect for improvising.
Do the same with A minor: leave out its 2nd and 6th (B and F again) to get A C D E G, the minor pentatonic.
And just like the full scales, C major pentatonic and A minor pentatonic use the same five notes: A minor pentatonic is the of C major pentatonic.
Add one note to the minor pentatonic, the lowered 5th (E♭ in A), and you get the blues scale. That extra "blue note" squeezes in between the 4th and the 5th and gives the scale its gritty, bluesy sound.
On a few roots the blue note is written as a raised 4th (♯4) instead, where a lowered 5th would need a double flat: in E♭ blues it's A, not B𝄫. It's the same note either way.
Guitarists know the minor pentatonic for its "boxes": five shapes with two notes on every string. Here is the first one, A minor pentatonic at the 5th fret, with the root notes filled in and finger numbers (1 = index … 4 = little finger):
See all five boxes, on any root note, in the fretboard tool (Scales → Fingering).
Next: Scales on the Guitar