Basic info:

Direct object pronouns replace the direct object of the sentence. A direct object is the thing/person that "receives" the action of the verb. When you read the verb, ask yourself "who?" or "what?" and the answer is the direct object.

I see you. She bought flowers. We saw the movie. You drank the water. She called me.

A direct object pronoun will replace the bolded direct objects above to turn them into the following:

I see you. She bought them. We saw it. You drank it. She called me.