Object

An object shows which thing is directly affected by an action. For example, for the sentence "The boy writes a letter," you can ask yourself "What is happening? There is someone who is writing." Who is writing? - the boy is writing. Therefore, "the boy" is the subject, the one performing the action. What is the boy writing? - The boy is writing a letter. Therefore, "letter" is an object, that which is directly affected by an action.

The ending -n indicates an object. You should put -n at the end of a word which indicates an object of an action.
La knabo vidas la hundon. - the boy sees the dog
La hundo vidas la knabon. - the dog sees the boy

La knabo la fiŝon manĝas.
La knabon la fiŝo manĝas.

You should also put -n at the end of adjectives that describe an object.
La knabo havas belan hundon. - the boy has a beautiful dog

You should also put -n after a plural ending -j for objects that are plural.
La hundo vidas la belajn katojn. - the dog sees the beautiful cats

More detailed explanations

Read here.

Exercises

Level 1, Level 2, Level 3