A knowledge of pitch intervals and ability to recognise them by sound is essential in learning music.
There are two groups of intervals – Primary – 4th, 5th and 8va (octave)
And Secondary – 2nd, 3rd, 6th, 7th
For ear training familiarise yourself with the sound of a major scale (C to C on the piano, white notes only) and start to recognise each note as 1,2,3,etc (“root”, 2nd, 3rd) – similar to “Doh, Ray, Me”.
The ability to recognise the distance between one note and another is important but I think that recognising a pitch relative to the root (Doh or 1 in the scale) is more useful and far easier! Trying to hear one interval quickly after another means that after a few notes you’ll be lost but if you keep the root note in your mind, then you’ll always have a reference point and it’s easy to hear the “pull” that a note has back to the root.
1 Cmajor (root / tonic)
2 Major 2nd
3 Major 3rd
4 Perfect 4th
5 Perfect 5th
6 Major 6th
7 Major 7th