People on here are doing the whole "NO SCAM! DO IT" kind of thing. I'd be careful as well, of course.
So how this works:
You go to CryptoTab's chrome app page. There you install the extension for Google Chrome browser. The only thing left to do is to choose with how much CPU power you actually want to mine.
Does it work? Yes. Does it go fast like GPU's and such? No. It's a small amount of BTC every now and then, but on the other hand you'd just not do anything and also get nothing.
I have it sit quietly on my pc all times at like 20%. When I'm away from my PC, may it be for work or such, then I'll put it to 100%.
Do the creator's of it make more money on it than they give you? Of course they do. But like I said, it's free and it actually works.
Just give it a whirl and see what it's about. As with all BTC mining you need patience. A lot of it.
I hope this clarified it a little.