What is a Machinima?
It's a weird word and you're probably pronouncing it wrong. But in short, machinima gets it's name from machine and cinema. It is the artform of taking a real-time 3D images and creating movies from them. Think of it as making a movie in the matrix. You have to light scenes, stage actors, dress sets and choose camera angles just like in real-life, but in machinima, you have the luxury of being able to do things like throw your character off a cliff without a safety net to catch them and they won't die or sue you.
Or maybe you want to have your main character drive a car down the highway in on-coming traffic. Not only will you not have to use a stunt driver, you can also rest assured that if you have a catastrophic crash, your main actor will still be ready for his close-ups. but maybe it'd just be better to share some videos to give a little more of a glimspe into what I'm talking about.
Another great source of information is the youtube channel, whanowa. He specializes in making machinima's using the 3d render engine from Rockstar's GTAV. His videos offer a ton of insight and depth if you're really interested in what all goes into making a machinima.