I am creating a game in c++ that will me multiplier and am going to post it on https://indiegamestand.com Here is one of the requirements "Must own the rights to distribute your game and to assign those rights to us (non-exclusively) for the purpose of selling here." If I compile my game as an .exe do I have that right? How can I make sure I retain it, and what can they do with the game if I give them that right?
You own the rights if you created it (source code and media content). As long as you don't have an existing contracts for work you create (sometimes tech companies will do this while you're employed with them, if the work is similar to the work you do there), what you create is yours. Also ensure the software and hardware you're using does not claim any rights to the content you're using them to create.
You can prove that you have the rights by showing evidence that you're the creator, with source code, notes, dated correspondence (emails talking about what you're working on maybe?), etc.
If you give them the right to sell it, they have the right to sell it. You're not giving them rights to the game, just rights to sell the game, and they're non-exclusive rights, so you can still sell it other places.