OCLC is like a big library of information about other libraries. It helps people find books and other things that they want to borrow or read.
When you visit a library, you might notice that each book has a number on it. This number is unique and helps the library keep track of where the book is located. The number is like the book's address.
Just like each book has an address, each library has its own address too. It's a bit like your house address, but instead of numbers and streets, the library's address is made up of letters and words.
When you want to find a book, you tell OCLC what book you are looking for and what library you want to borrow it from. OCLC looks up the address of the library and the address of the book, and it tells you where to go to find it. This process is called an identifier, because it is helping to identify the book and the library.
So OCLC's identifier is like a really helpful wizard who knows the address of every book and library, and can help you find what you're looking for.