Intervals are the distance between two notes. The most commonly used by fingerstyle guitarists are 6ths and 10ths. Play a C major scale beginning on the 3rd fret of the 5th string. Count as you play, giving a number to each note in the scale. The distance between the 3rd fret of the 5th string and the 2nd fret of the 3rd string is asixth. Lower the note on the 3rd string by one fret and you have a minor 6th.
Do the same thing again, but this time keep going until you get to the 5th fret of the 2nd string. This is a 10th. Lower the note on the second string by one fret and you have a minor 10th.
The 10th is great for fingerstyle guitarists to use. It sounds like a big chord but it only uses 2 notes. This makes it easy to grab so it's great to use in fast moving parts of a song.
The examples below shows how to play scales using 6ths and 10ths. There is also a short song using 10ths to show them in action.