When you call a function in JavaScript, you tell the program to execute the code inside that function. To call a function, you write its name followed by parentheses. If the function expects data, you provide it inside the parentheses. This process is called invoking the function. When a function is called, the JavaScript engine jumps to the function's code, runs it, and then returns to where the call was made. You can call a function as many times as you want, and each call can use different data.