if the game based on collectables then why should be there a theme i'm so confused anybody can just start on the game and change the story to fit the theme

no

the main topic of the jam is collectables but they will choose the theme, so, the game must have collectables but with the theme of the jam that will appear on the jam's page(example: the theme is "cars" so you have to do a car game(it is just an example) and in your game u should be able to collect something cuz the jam MAIN theme is collectables) hope u understood

Collectables are the limitation. The game must feature them to qualify for the jam. 

The theme will be a word or phrase to spark creativity as a secondary requirement. 

And no, you shouldn't start before the jam begins.