Please Help !!!! Lol

if this was already posted, please excuse me....

I have an 2002 optima, and i think my battery is out. I went to undo the battery and noticed on the side a small black plastic box with a square fuse like thing coming out the top. I triied to undo the fuse thing thinkng, maybe this is the problem. I forced way to hard and broke the fuse thing. What is this fuse thing? Still need to buy some tools so i can take out the last bolt to remove to battery to replace it. This is how much i need help, i don't even own tools..lol.... Can someone please advise me.

Thank you in advance !!!!!