One note played alone is termed a “unison.” Two notes played simultaneously  or adjacently are called “intervals.” Three or more notes in combination are termed “chords.”

So if C and D are played simultanously the distance between them is called “a 2nd.” C and E would be a 3rd, C and F a 4th, C and G a 5th, C and A a 6th, C and B a 7th, and C and C an octave. (Octaves — notes with the same name — are considered as a type of unision.)

Intervals also have different flavors — there can be major intervals, minor intervals (like C and Eb), diminished intervals (like C and Gb), augmented intervals (like C and G#), and perfect intervals (like C and F and C and G — so called because they have a perfect relationship — they are in each other’s scale).

To learn more about intervals go to https://www.playpiano.com/Articles/unisons-intervals.htm

So intervals simply measure the distance between any two notes. There are two types of intervals: consonant and dissonant. The most consonant intervals are 3rd and 6ths, while the most dissonant intervals are 2nds and 7th. 4th and 5th are somewhere inbetween — depending on the context.