Truth or dare is a game where you are allowed to choose between either. If you choose dare, and for some reason you are not able to complete the given task in hand, they will force upon you to do something or the other. That's what happened with Rob. He lost, so his friends told him to get an embarrassing tattoo to go with.
However, I would like to highlight, it is not necessary for you to be drunk to play this game. It can be played anyhow. Sometimes it is played along with spin the bottle to choose who is to be asked/provided with the options.
The penalty upon loss is decided randomly (depending on your capability to do the job).