Answer:
Each fundamental magnitude has its measure. The distance is measured in centimeters or meters (or inches and feet). The mass is measured in grams (or pounds). Time is measured in hours, minutes and seconds. In music, we also have a resource to "measure" the distance between the notes: the intervals.
Intervals are the measure we use to measure the distance between the notes. This distance is measured by counting the number of notes between two notes given. For example, if we have the Do and Fa notes, we say that there is an interval of a quarter, since between Do and Fa there are four notes (Do-Re-Mi-Fa). Another example: between Re and La there is a fifth, since there are five notes between them (Re-Mi-Fa-Sol-La).
The intervals have a very specific classification, according to the amount of semitones or tones they have.