What a major pentatonic scale is
In most practical music contexts, the major pentatonic scale means the five-note form 1, 2, 3, 5, and 6 from a major scale. In C, that is C, D, E, G, and A.
It leaves out scale degrees 4 and 7, the two notes that create the strongest half-step resolutions in the major scale. That is why the sound feels open, consonant, and easy to place over many major-key chord progressions.