A prototype is your way of testing game concepts without investing much time or effort. This way, you can imagine if a game idea is viable before you put the time to code it, make art, add music, etc. Most teams will be submitting a prototype of a game rather than coding a whole new game from scratch. You need to make sure the prototype is playable and that it portrays your game elements effectively.
The following resources are intended for those designers with little or no experience programming. Feel free to add more resources in this thread.
Tools that do not require coding:
Twine. Choice-based narratives, adventure games, RPG games.
Emotica by Anna Anthropy. 2D maps, world exploration, escape rooms.
Phaser Editor. 2D platformers
Stencyl. 2D platformers
Board game prototyping using digital tools