You have Soul Bonded with one of your allies. This means that your hit points are interchangeable. As a bonus action on your turn, as long as you are 60 feet from your Bonded ally you can reduce your hit points by any amount from the hit points you have left, and your Bonded ally will gain the exact same amount of hit points, and they can do the same for you on their turn. 

Additionally, if your Bonded takes damage from an attack, you can use your reaction to take the damage instead and reduce the damage they take to 0 if they are within 60 feet of you. Your Bonded is able to do the same. If this ability is used, the resistances and vulnerabilities of the initial target is used for damage calculation.

To get this Feat, another creature, usually another party member, must take this Feat at the same time that you do. (If xp is uneven so that one character would get it first, then the higher level character can hold back on earning the effects of this feat until the lower level character gets it as well)