"Please do not mind but what is bitcoin mining?"
Bitcoins are a form of digital currency. You can "mine" for them by running computations on your computer that help encrypt the whole system to make it safer. Video cards are more efficient than CPUs for the process, and some video cards more than others.
I spent $150 on a video card and leave my machine running when I'm not using it. I'm only making about 1 coin a month, which is about $20 last time I check. It's probably close to breaking even with my added electricity bill. When I started I was getting about 1 bitcoin a week, but production decreases over time. My computer puts out a lot of heat now with the video card running full steam, so I figure I am just subsidizing my heating bill. It's nice in my office now in the winter. Maybe not such a good idea in the summer.
As other people pointed out, you either need to take it to the next level to make it profitable or just buy/sell bitcoins because the price fluctuates a lot.