This is called a possessive apostrophe. Let's take a look at some examples. The cat's tail was fluffy. Cat is a singular noun so you need to add an apostrophe and "s" to show that the tail belongs ...

A singular noun is a noun referring to just one person or thing, for example 'cat'. When you want to show that something belongs to that person or thing, you add a possessive apostrophe and an 's'.