The Independent Spirit Award for Best Film is an award given to a movie that was made outside of the big Hollywood studios, by a team of people who are doing their own thing and not following the rules of mainstream filmmaking.
It's like when you and your friends make a cool project at school without your teacher's help. You didn't do it the way you were supposed to, but it turned out really awesome and everyone thinks you did a great job.
The Independent Spirit Award is like a big gold star that says, "Wow, you guys did something really special and different!" It's given out every year by a group of people who really love and support independent movies.