Arguments

Functions are a way to provide input into an algorithm and to get output based on that input. In programming, functions usually look like this when you "call" the function.

// This is an `add` function that takes input (arguments)
// and returns the output (5 in this case)
add(1, 4)

We're calling the add function and the parenthesis allows us to pass input with commas separating them. These inputs are called "arguments". The function name is "add" so we can derive it's just going to add the argument numbers together and return 5 in this case.

Sometimes when we look at a function name, we can't tell what the input should be and what the output would be. So we look up the documentation for how the function works. The documentation will give us information on interface known as an API which is to say it will tell us what arguments we are allowed to pass in and what output we might expect.

Depending on the programming language, sometimes we can write our own functions like in JavaScript. But CSS doesn't let us write our own. But we can "call" the functions that are built-in to the CSS language. For example, CSS has an rgb() function to create an RGB color.