Rabbits can get fleas from a number of sources, principally other animals and material that is infested with flea eggs. If your rabbit is scratching a lot, then they could have fleas - they will need to be dealt with quickly, as they are very uncomfortable for pets and can be transferred to humans!
Fleas are nasty little insects that bite - they are unpleasant for both humans and rabbits
If your rabbits live outdoors, then they can pick up fleas from cats that roam throughout your neighbourhood. They can also pick up fleas from other rabbits - if you introduce your rabbits to some new friends, it’s best to be sure that the other owner has thoroughly checked their pets recently.
If you have any other animals in your household, such as cats and dogs, then it’s wise to give them very regular flea treatments, both to minimise the chance that they themselves suffer, but also to minimise the risk that they transmit any fleas they catch to your rabbits.