What Are RESTful APIs?
RESTful APIs are those APIs which follow some set of Instructions and constarints while developing APIs.
Too soon?

What Are APIs?
API stands for Applilcation Programming Interface and APIs are a set of instructions and set of routines for the computer to follow to exchange information between two or more services which are running independently or at the same. Also, for the communication between Frontend and the Backend, APIs work with most programming language for instance Java,C++,C#, Java Script etc.

Take this for this example , think of a restaurant.

You visits and tells the waiter for which food you like to order, the waiter now takes your order to the cook and tells the cook of what the customer needs and wait for the good to prepare the dish and after the dish is prepared the waiter takes the food back to the table and serve.

Now, in the example, the customer is (Client or Frontend) requesting for certain resources(data),

the waiter(our API) takes the response and goes to the Cook(Backend) and the cook gather the ingredients from Storage Room(Database) and gives the food(data) to the waiter ,and the waiter previews the data back to the client

https://erpsolutions.oodles.io/developer-blogs/An-Overview-of-REST-...