This mod limits you to a set number of lives per world. By default, when you die, you will lose half a heart of your permanent health, leaving you less and less hearts/HP after every respawn. When you finally run out of lives, depending on the config, you will be forced into Spectator mode (not available for MC 1.7.10) or banned from the server for a set amount of time.
Use the command /ll or /limitedlives in chat to find out how many lives you have left or how long you have left to respawn.
To set a death count, do /ll <set> <player> <deaths>
Versions older than 1.0.x (MC 1.12 and older) uses chat commands, so !ll or !limitedlives.