Plainswalkers are a new type of card. They come into play with X counters on it, which is shown in the bottom right corner. Each one has 3 abilities, one that adds a counter, one that removes a low number of counters, and one that removes a lot of counters. On your turn you may use one of those abilities, adding or removing the appropriate # of counters. The plainswalker dies when it has no counters.
The opponent, when attacking, can decide which creatures attack you, and which creatures attack the plainswalker. Any damage that gets through removes 1 counter per point of damage. Any spell that deals damage to you can be directed at the plainswalker as well.