The intervals are the distance between two notes. - means down and +
means up. There are major and minor and perfect intervals, and here's
the list:

Perfect unison (U)
minor 2nd (m2)
Major 2nd (M2)
minor 3rd (m3)
Major 3rd (M3)
Perfect 4th (P4)
Tritone (TT)
Perfect 5th (P5)
minor 6th (m6)
Major 6th (M6)
minor 7th (m7)
Major 7th (M7)
Perfect Octave (P8)

It goes on, 9ths and 10ths etc, etc, etc. But those are the most common
intervals, as we don't usually go over an octave. Make sense? It's
easier to demonstrate on piano.