This is a standing joke at most engineering schools and it explains what it is in a very short way. Recursion is a mathematical concept. Let's explain it a bit more. The official definition says the following:
Ok, what does that mean in human speak? It says that at some point in running our function, we will call ourselves. This means we have a function that looks something like this:
function something() { statement; statement; if(condition) { something(); } return someValue;