The leave in for hair
is a leave-in conditioner that leaves your hair soft and easy to comb.
Translating literally from English, we are talking about a “leave-on” treatment, a treatment that therefore does not require rinsing.
Leave-in is a light conditioner, to be left on after washing, a treatment that replaces (or supports) the conditioner.
Its purpose is clearly to condition the hair, therefore soften and moisturize it, but it also has a disciplining function, so it could easily be comparable to a moisturizing / modeling cream or a disciplining / polishing oil, such as the famous liquid crystals.