In anting, a bird spreads ´ant fluid´ over its body and feathers. It either does this by letting the ants crawl on it, or by grabbing the ant in its beak and rubbing the insect on its body.
The acids in the ants helps fight off the mites that live in birds´ feathers. They typically do the anting, and then preen and clean themselves.
Birding Glossary