What is a "Render File"?

FYI: I’m a computer dummy that’s why I use an iMac. I can do a lot just knowing how to point, drag and click. Anyway…

While deleting movies and photographs from my iMac (macOS Mojave 10.14.4) I see files called “Render Files”. What exactly are they?

I use “OmniDiskSweeper” to delete old files (movies and pictures) because draging them to the trash can and emptying it does not remove the files from my start up disk.

I love making movies with iMovie and my GoPros (I have 8 of them). Of course I have to continually manage my startup disk storage space. I siphon off the movies that I want to keep to YouTube and or a 4 terabyte external hard drive. Been deleting a lot of Render Files. Thought that it might be a good idea to try to understand what they are.

Tried googling Render Files but didn’t find a definition.

Thanks!