An Interval is the space between two notes, like a mile is the space between two places.

There are 8 notes in a standard major scale. Here C major is an example.

1 2 3 4 5 6 7 1

C D E F G A B C

If C=1st what is G? Ans 5th.

If A=1st what is a 6th? Ans F (In this question you should realise that we are now in A minor, C majors relative minor. See Key Signatures to understand this)

So to understand music intervals you have to know your alphabet up to G & be able to count to eight.

That's it!