Good question! You can get health kits 2 different ways.
You can knock out a Guard and they will drop a health kit along with the weapon that they are holding. Or, you can defeat another Player and pick up their health kit!
Health kits heal 50% of your max HP!
You can only carry 2 health kits at a time, so make sure that you use them as soon as possible so that you always have space to pick one up!