What does major scale mean in music?

In most music, the majority of the notes in a tune are a set of seven notes chosen from the 12 notes of western music. The "major scale" is one way of selecting these seven notes. In the key of C, the major scale is C-D-E-F-G-A-B-C. The song "Do-Re-Mi" is all about the major scale. If you can sing that, then you know the major scale intuitively.

As an example, the main Star Wars theme uses a major scale:

C G F E D C G

The "Jedi" theme does not use a major scale; it includes e-flat which is not in the major scale.

G G C D E-flat F E-flat G

Note that the notes of the major scale are not evenly spaced in frequency. The interval between C and D is twice as big as the interval between E and F. The intervals (in terms of semitones) is:

C(2)D(2)E(1)F(2)G(2)A(2)B(1)C