Summer Internships are sort of like work placements that accounting/consulting firms and banks etc offer, mostly to students in their penultimate year. You can apply for them early/during second year and there's a long selection process. If you are successful, you get to work in a section of the firm during the summer holidays, getting some experience in the industry. From what I've heard and know, it's pretty useful to try and get one - not only does it give you some exposure to the industry and looks good on your resume, but also if they like you enough they'll offer you a grad position, saving you the time and effort in third year. I would say that yes, in terms of these things marks would matter and they'd be the basis for the first round of culling applications, but they're not the only thing that the firms will consider, they want well rounded people.
Agree with what charmanderp said, if you think you can get 80+ why not go for it? It'll probably come in handy somewhere later on the track, could boost your average etc. Just try your best.
I wouldn't say you are "screwed" if you get bad marks or fail, I know of some people who have got vac work having failed a subject but obviously I'd be making more of an effort next semester to try and pick your grades up. (But there's still time, you won't necessarily be screwed as you might think)