Feeder fish are just that. Fish that you either buy to feed to your axie or breed yourself for the same purpose. Some things to consider....
Purchased feeders are generally disease magnets due to the lousy conditions in which they are kept. As such, they should be quarantined for a minimum of 30 days before going in with another animal. Feeders you breed are different as you can maintain the tank to higher standards, feed them nutritious foods, and ensure they are healthy before offering them up as food. Some people, however, find that with all the care they put into raising them they are more like pets and can't bring themselves to feed them to another pet. Additionally, some species of fish will harass the axie, nit its gills etc. That will create stress and stress has a very large impact on overall health so you may have an axie more prone to illness. Lots of ways to skin the cat though so read up and choose what is best for your situation.
Personally, i'd offer a variety of commercially available foods suplimented with earthworms.