There is basicly a flap that you need to open soo that you can put gamecartridge inside. The flap is there to prevent dust and other things from getting in that slot. You can put a game in there at all time to prevent dust and other things from getting in there if you don't have the flap.
#is the card holder slot not closing a bad thing?
tough plinth
wide orchid
I'm assuming you mean the game card slot. I would worry if it doesn't close as it won't affect the performance of the console.