I got a major question

You don't seem to be a coder, so I'm gonna guess you're trying to run some game you downloaded the source of. So either you're missing some files, or some files are of an old version, or the source as a whole didn't compile before you downloaded it. Best point to start for you, is the original source you downloaded and see if you can run that.

im trying to use unity to make a game, im getting those errors. and im not used to these codes, i just got unity and im trying to learn it. But im trying to test my game, and it says i have to fix the errors before i can do it. but idk how to fix it. I know about the red dot at the bottom left, you double click that etc.. idk what to put to fix it. i've been looking into it on this site and there are answer, but i cant understand them. thats what im asking. Something a 4yo can understand.

If you could paste some code in your question (edit it), it might help, but stijn is probably correct, you've probably got corrupt or incomplete file, or old ones. Or ones that never were tested.

Im slowly getting used to this. But, I deleted all my assets from the unity project tab, and added one at a time. The first thing i imported was terrains. It got errors. They say "terrain splat 1 is null" "terrain splat 0 is null" "a tree couldn't be loaded because a prefab is missing".

and thanks for all your help everyone

and if i do have corrupt/incomplete/old files, how do i fix it?