Hi Micha,
The intervals lesson is part of a 2-part series of lessons called "guitar fundamentals."

Prior to the intervals lesson is another one about how the notes are named. It is here at the end of guitar fundamentals 1:

http://www.guitartricks.com/lesson.php?input=11078&c_id=1&ch_id=5&s_id=1138

The intervals lessons are at the beginning of guitar fundamentals 2, which assumes you have already taken all the guitar fundamentals 1 lessons.

So if you haven't taken guitar fundamentals 1 yet, you should probably start there so things make more sense.

Hope that helps?

Jon