Watch the Intervals online guitar lesson by Dmitry Lisenko from Percussive Acoustic Bass (Foundry)
An interval is the distance from one note until the other. If I take a one note and then a second, there will be the distance between them, the higher or the lower the second note, the bigger distance is between them.
We can count distance by using major and minor scales. Just by going from start till the required fret. By playing intervals together, we can get much fuller sound, and create more interesting compositions.