Nouns can be either singular or plural.

If a noun is singular it means that there is only one of them, but if it is plural there is more than one.

one cat (singular)

three cats (plural)

We make many singular nouns into plural nouns by adding -s, but if a noun ends in -f or -fe then we have to take off the -f or -fe and add -ves instead.

knife / knives

wolf / wolves

The word roof is a big exception to this rule. Even though it ends in -f we just add -s.

roof / roofs

If a word ends in -ff then we just add -s.

cliff / cliffs