First aid will determine how effective a character will be when using first aid kits. Characters with high first aid skill will be able to use a first aid kit to it's maximum effectiveness, and is likely to increase health in one healing. Characters with low first aid skill will not be as effective when using first aid kits.
First aid skills can be increased by healing characters during missions. Even characters with low first aid skills are able to increase their effectiveness, though this may take some time, and a lot of first aid kits too.