Basic info:

Indirect object pronouns replace the indirect object of the sentence. An indirect object is the PERSON for which/to which the action of the verb is done. When you read the verb and notice the direct object, ask yourself "to whom?" and the answer is the indirect object.

He gave flowers to me. We sent a card to grandpa. You told the lie to Becky and me.

An indirect object pronoun will replace the bolded indirect objects above to turn them into the following:

He gave flowers to me. We sent a card to him. You told the lie to us.

Sentences with an indirect object almost always have a direct object, although it it may be implied.

She wrote to me every week. <-- Here the direct object is absent, but implied. We can assume she wrote SOMETHING to you--a letter, a text, a joke, etc.