When killing another player you get one of their lives. Lives can be exchanged with heart items you can trade with other players or consume to get the life back.

There is a command to withdraw your hearts: /withdraw amount
Hearts can be crafted and the default recipe for crafting a heart is an enchanted golden apple surrounded by diamond blocks on the edges and netherite ingots on the corners although it can be changed in the configs together with max lives and max lives message