It’s the distance (interval) between two notes, equal to two whole tones.
We can name each interval and enharmonic position in the chromatic scale relative to the root. Let’s say C major for example.
C = Root = Perfect Unison
C# = Augmented unison
Db = Minor 2nd
D = Major 2nd
D# = Augmented 2nd
Eb = Minor 3rd
E = Major 3rd
And so on, I won’t list them all here, they’re easy to look up.
In the major scale we usually number the intervals from 1 to 7. If we called them by their intervalic names it would be
1 = Perfect unison
2 = Major 2nd
3 = Major 3rd
4 = Perfect 4th
5 = Perfect 5th
6 Major 6th
7 = Major 7th
Major chords have a major 3rd interval but we generally just refer to it as the “third”.
If we drop that note by a semitone then it becomes a minor chord because the interval between that and the root (C > Eb) is now a minor 3rd.
That’s a quick explanation but hopefully answers your question.