I've seen quite a few non-technical people look worried just at the mention of the word API. Possibly due to not understanding what and API actually is.
API's aren't scary. They are just made up of questions and replies.
API stands for Application Programming Interface but this doesn't tend to mean a lot to non-technical people.
At a very basic level, the term API is used to describe both sides of computer systems that talk to each other using language which is pre-agreed and understood. Software can make requests using APIs in order to receive responses and this is why API's are like take-away menus
Imagine the menu for your favourite take-away restaurant......
That menu is like an API between a customer and the restaurant. It is something that lets both side communicate with each other.
When a customer phones the take-away and places an order from the menu, imagine they are software that is making a request to an API. When the restaurant delivers the food, imagine this is like the customer receiving a response from the API.
The takeaway menu does not include the recipes and instructions on how to cook the food. The customer does not need to know how to make each dish (because the restaurant takes care of this). The menu simply lets the customer select the food they want to eat from the choices provided by the restaurant.
The recipes and instructions on how to cook the food would be like source code. APIs make it possible for software to share information and interact with each other without having to share all their code.
If the restaurant changes their recipes or ingredients, the menu will still work and the customer will still be able to order.
So if the source code changes for a piece of software, the API will still work.
If each dish on the menu is numbered (like at a Chinese take-away) and the restaurant changes the numbers, a customer with an old menu ordering using the old numbers will receive the wrong meal!
Changing the numbers on the menu is like changing an API which is already in use. This should be avoided if at all possible.