Two notes play, you name the distance between them. Interval recognition is the foundation of relative pitch: it's what lets you work out melodies, bass lines and solos by ear.
Level 1 uses only the major third, the perfect fifth and the octave. New intervals join level by level until all twelve are in play at level 5. From level 6 both notes sound at the same time, which is a real step up: you have to hear the interval as a single color instead of as a movement.
Work with a few very different intervals first and add their neighbors only once those are reliable. Short daily sessions beat long occasional ones, because recognition is a reflex you build, not a fact you memorize.
They help at the start (many people remember a perfect fourth through a familiar tune), but the goal is to recognize the interval directly. Once a reference has done its job, stop relying on it.