Loyalty is a characteristic of a Planeswalker, as introduced in Lorwyn..
Although mechanically similar to life, it is flavorwise not the same.   Damage doesn't kill the planeswalker, it just encourages him or her to stop helping you.
From the Comprehensive Rules (Archenemy: Nicol Bolas (June 5, 2017))
- 209. Loyalty
- 209.1. Each planeswalker card has a loyalty number printed in its lower right corner. This indicates its loyalty while it’s not on the battlefield, and it also indicates that the planeswalker enters the battlefield with that many loyalty counters on it.
- 209.2. An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if none of that permanent’s loyalty abilities have been activated that turn. See rule 606, “Loyalty Abilities.”
- Damage dealt to a planeswalker causes that many loyalty counters to be removed from it. This is a damage result; see rule 119.3c.
- A planeswalker with loyalty 0 is put into its owner’s graveyard. This is a state-based effect; see rule 704.5i.
The number varies.
- ↑ Mark Rosewater. (November 05, 2007.) “Planeswalk on the Wild Side, Part I”, magicthegathering.com, Wizards of the Coast.
- ↑ Doug Beyer. (August 26, 2009.) “Your Mailbox is Over Vorthosity”, magicthegathering.com, Wizards of the Coast.
- ↑ Doug Beyer. (November 25, 2009.) “Brooms, Planes, and Valakuts: Tales from the Inbox”, magicthegathering.com, Wizards of the Coast.