A Life meter is a part of the HUD that shows the player their characters health. Sometimes called a Life gauge, health meter, or even life bar, the life meter lowers when damage is taken and increases when a sort of power up or medical item is used. Usually when the life meter reaches zero (or in other words becomes completely empty) that character either dies, is knocked out for the round, the game is over, or they must respawn. The way a empty life meter is handled depends on the type of game that is played.
Most of the time the life meter is in the form of a bar that shrinks or rises (snicker...) but can also take other forms. For example in the game The Legend of Zelda the life meter is represented in hearts and the maximum can be increased by finding heart containers. In the example image to the right you can see the 3 hearts you begin the game with and after getting a heart container it will then increase to 4.