Crits and Misses are the soul of TF2Tags. They are what every single one of the site's users strives for every day. Whenever you click either one of those buttons beside a newly-submitted item, you are either validating their existence or crushing their hopes to dust.
Well, okay, that was a mite overwrought. They're still the backbone of the site, however.
The Science Behind Rating
Crits and Misses are the site's own upvote/downvote system, enabling users to 'crit' items that they like, or 'miss' items that they don't like, in keeping with the whole Team Fortress 2 motif.
Theoretically, items that are funny or cleverly written are rewarded with crits to the submitting user's name, providing pretty much the entire motivation for using this site. No, really. It certainly sounds shallow when you put it into words, but IT'S IMPORTANT TO US, DAMNIT.
When you submit an item, they appear on the Latest page with a small box on the side labelled 'Rate This Item'. An example is shown at right.
From left to right, this box shows the item's 'points' (total crits minus total misses), the voting buttons, and the item's 'K/D Ratio' (total crits divided by total misses).
Critting an item adds 1 to its points, while Missing it subtracts 1. And thus, the foundation for a dick-measuring contest is laid for eternity!
The Importance of Two Buttons
Well, why the hell would you submit items otherwise? (That's a rhetorical question, of course. The obvious answer is shitposting. And nobody likes shitposters, unless they're AFI.)
To give you a proper answer, though, the points you get from items are awarded to (or deducted from) your personal profile. These points go towards your rank, and your general social standing within the community.
If your item gets enough crits, it could get into the Top Rated lists, setting your accomplishment in stone. Well, at least for 30 or 90 days. (Every item is technically in the All Time list, so that doesn't count.)
And... well... I guess lime numbers are attractive? Wait, everybody in TF2 hates lime. Damn, this is hard...
Oh, except Tryhard. He hates lime on anything except his aspect. Good on him.