Definition of walk
A walk in poker is something that can only happen to you when you're in the big blind position. A walk is when all the players before you have folded and therefore you win the blinds without even playing the hand. Since all the players have folded before you, there is no need to show your hand, you just collect the jackpot.
The walk in tournament
You will probably have more walk in tournaments than in games of cash games. For example, if you have a very aggressive tournament image and you are the leader of the chip table, it's quite possible that you will have a lot of walk's. Players, with no hand, won't dare to steal your blind, they will think that you are very aggressive and won't let you. Also, because you will have a big stack, you will be more likely to call. It's a good reputation to have in this situation since you may not always be able to defend your precious blinds in tournaments. A walk is always a good thing when you're in the big blind position (except when you have a monster, of course).