@als.gillis123 In much of Western music theory things are explained in relation to the major scale. For example C major scale is C D E F G A B and these 7 notes can be exchanged for the numbers 1 2 3 4 5 6 7.

The basic structure of common chords are triads with 3 notes which are called the 1st, 3rd and 5th. If you take note numbers 1 3 and 5 from the scale of C major we looked at above you have C E G.

The distance between C and E is called a third and the distance between C and G is called a fifth. Exactly the same as how the distance between 1 and 3 is a third and 1 and 5 is a fifth.

Play C and E together on the guitar and you are playing the interval called the third.

Now there are actually two types of third; major and minor. C to E is a distance of 4 semitones/frets and it’s called a major third. D to F is a distance of 3 frets and is called a minor third. Both of them are still the third because we took that C D E F G A B scale and play one note, skipped the next note then play the next note (we’re not talking about semitones/frets but notes of a scale).

A third which is 4 frets away is major and a third that is 3 frets away is minor. The logic is not important. Take your guitar and play notes C and E together and then D and F together and just get used to those sounds and how different they sound.

This course explains this and much theory https://worldmusicmethod.com/courses/theory-by-ear/