The ants marched in a single file for food.

Meaning: This sentence describes how a specific group of ants are moving in a straight line, one behind the other, to find food.

Choice of Words: The is used to refer to a specific group of ants. Ants are small insects that live in colonies and work together. Marched means to walk in a straight line, usually in a group. Single file means to walk in a line one behind the other. Food is anything that an organism eats to get energy. So, when you put all these words together, it means that a specific group of ants were walking in a straight line, one behind the other, to get food.