All nouns in Latin have case. The case of the noun tells you the function of that noun in the sentence.
Nouns that have nominative endings are generally the subject.
Nouns that have accusative endings are generally the direct object.
Here is a chart that shows you the endings.
It is very important to remember that a noun can never change gender or declension (which is a group of nouns with a similar pattern of endings). So, any noun that ends with -a in the nominative can only have -am as its accusative ending. In addition, any noun that ends with -us/-er can only have -um/-rum as its accusative. They are not interchangeable.
When a noun becomes a direct object, it changes to accusative. When a noun changes back to the subject, its endings become nominative.