Answer:
Harmonising is when a note is matched or harmonised by another note.
Most vocal harmonies harmonise using 3rds, 4th, 5ths and Octaves.
Learning how to Harmonise can be a chore because you'll want automatically sing along with the main harmony as it's the stronger part.
If you have a piano play just play a C and then play an E and hear the birght happy relation they have, and then sing/hum the C and E relation and then try and play a C and then hum the E without the piano playing the E.
For more harmony practices,
C - E = Major 3rd
C - D#/Eb = Minor 3rd
C - F = Perfect Fourth
C - G = Perfect Fith
C - C = Octave
Practice makes perfect, try and listen to some of your favourite bands with back up singers and try and hear the harmonies. It might be hard at first since the back up singers aren't as loud in the mix of the music compared to the lead vocalist.