Pentatonic is a five note scale from "Penta" meaning the number five, and "tonic" meaning a set of musical notes in this case.
As mentioned on the first page: Click on the All Notes - No Color button to see all the notes. Then click on Piano Fretboard to see the sharp notes (like the black piano keys). If we take out the black notes, we are left with the key of C Major. The black notes form a pentatonic scale.
The black notes contain a Minor and Major Pentatonic scale, depending on which note you start from. It's the same five notes but the Minor Pentatonic goes well with Minor chords, while the Major Pentatonic goes well with Major chords. Like the Major Scale and it's Modes, you get a different sound depending on where you start.
See the next page for the E Minor Pentatonic Scale.