Interval degrees describe the number of scale steps from the Tonic (scale's 1st step).
For example, a Major 3rd interval, represents the distance of the 3rd scale degree away from the Tonic:
In the same way, a Perfect 5th interval, represents the distance of the 5th scale degree away from the Tonic:
Any of the basic intervals (within the 1st octave), can be extended by an octave or more.
For instance a Major 2nd interval can be extended by an octave, to become a Major 9th.
In the next few lessons we will go over the basic intervals up to an Octave.