Minecraft wiki entry for Predicate, associated text: Target selectors: The selector argument predicate= checks predicates as a filter for entity selection. The predicate is invoked once per entity that needs filtering, each time being at the entity's location.
/execute: A subcommand, /execute if predicate, can invoke a predicate to either return a result or to decide whether to continue with a subcommand chain. The predicate is invoked once at the current contextual position of execution.