Object pronouns are the receivers of action in a sentence. They include me, you, him, her, it, us, and them. The subject in a sentence is the entity (person, thing, object or place) that is doing something. When a pronoun (I, You, He/She/It, We, You, They) takes the place of a subject in a sentence, it becomes a subject pronoun.
The object in a sentence is the entity (person, thing, object or place) that is being acted upon. When we replace the object (a person or a thing) with a pronoun, we use an object pronoun: I bought a car. I like it.
Examples:
I (subject) am driving a car (object).
I (subject) am calling him (object pronoun).
We (subject) are going to see them (object pronoun).
She (subject) will message you (object pronoun).