Similar to perfect pitch, you can recognize a note given that you have a reference note first. So, instead of listening to a tone and immediately go ‘It’s a G’, you’ll need to listen to another tone first and know what it is. So, this ability is mainly based on intervals, rather than the tones themselves. It’s more useful with chords, as the harmony -ie. the blend of notes- is perceived and identified as a whole, rather than identifying each note individually. So, people with relative pitch have arguably an advantage when listening to E-G-C, because as soon as they know that the lowest tone is an E, they go ‘It’s a first inversion of C major’. While, someone with perfect pitch would think ‘The lowest note is an E, then I’m listening to a G and the uppermost is a C, so it has to be a C major in first inversion’. Both have their advantages and disadvantages. Both can be developed over time with a lot of work. Or, in case you are lucky, you’re just born with the one, with the other, or if you are reaaally lucky, you’re born with both.