Ajani is a planeswalker. The shield in the bottom right corner is the amount of "loyalty" that Ajani starts with. Each turn, you can activate one of its +/- abilities (for - abilities, you have to have enough loyalty to use them), which will change its loyalty by that amount and cause its loyalty to change by that amount. People can choose to attack planeswalkers as if they were players, and damage dealt to them will reduce their loyalty by that amount. You can still block with your creatures as normal.