A major scale is a set pattern of notes which produces the familiar sound:
This pattern is based upon a set sequence of tones and semitones. A semitone is the smallest interval on a piano keyboard. E.g., the distance between a black note and the white note immediately to its left or right. A tone is two semitones. The interval between the notes B and C, and the notes E and F is always one semitone. The interval between all other natural notes (i.e., not sharps or flats), is one tone, as summarized here.
A tone is sometimes referred to as a step, and a semitone as a half-step.
The most common scale is called the major scale, which is based on the following pattern of tones and semitones.