Simply put, an interval is the tonal distance between two notes. Intervals are described by terms like half-step, whole-step, third, fourth, fifth,...etc. This is easy to see on a piano keyboard. Two adjacent keys are a half-step apart, two keys with one key in-between are a whole step. To name a "numbered" interval, count the number of notes (within the key signature of the starting note) from the starting note to the ending note.... WHAT?? 

OK...Think about it this way:

Find middle "C" on the piano. Counting this note as "one", count up five white keys to the note "G". (C, D, E, F, G...1, 2, 3, 4, 5). These two notes (C & G) are an interval of a "fifth" apart.

Sorry for the long-winded music theory class. I hope this answers your question.