Object pronouns are those tricky little words in sentences that replace nouns affected by verbs. There are two types:
1) Direct object pronouns (pronoms objets directs) replace the people or things that receive the action of the verb in a sentence.
2) Indirect object pronouns (pronoms objets indirects) replace the people in a sentence to/for whom the action of the verb occurs.
In addition, adverbial pronouns work in conjunction with the object pronouns:
Y replaces à (or another preposition of place) + noun
En replaces de + noun
Reflexive pronouns also come into play, particularly when trying to figure out word order for double object pronouns.
It is important to understand each of these concepts, because they are very commonly used and without them there is a certain "bulkiness" in French. Once you begin using object and adverbial pronouns, your French will sound a lot more natural.
Use these links to learn all about object, adverbial, and reflexive pronouns, including how to use them and correct word order.