What is a complete subject, simple subject, and a verb?

A complete subject contains the noun along with a group of words that describe it and a simple subject is only the noun.

A verb is the action word in a sentence.

The yellow colored balloon flies in the sky.

Balloon is the simple subject and the yellow colored balloon is the complete subject.

The yellow balloon flies in the sky.

Here "flies" is the verb that tells about the action.