Hit points are define how much damage your agent can sustain before becoming incapacitated (bleeding out). Once an agent's HP reaches zero, they will fall to the ground and turn timer will begin indicating when they will bleed out. They must be stabilized or extracted before this timer reaches zero.
HP is also important when performing a silent take-down. Take-downs cannot be performed on an enemy who's HP is higher than the agent who is attempting the take-down. For example: If DEADPAN's HP is 88/101 he will be unable to perform a silent take-down against an enemy agent who's HP is 95/95.
Once an agent's HP reaches zero, they become incapacitated and enter a "bleeding out" stage. The agent will expire based on the number of turns they have stamped on their avatar. To save the agent, you must use another agent to stabilize them, then carry the downed agent to the extraction point. If successful the downed agent will be able to be healed in the Infirmary. If you fail the agent will be captured by the enemy which could lead to the agent turning sides or revealing information about your operations.
Using Medkits you can heal yourself or other agents.
An friendly incapacitated agent must be stabilized to prevent them from bleeding out. A timer appears over the agent's avatar as well as a small icon next to their HP indicating how many turns they have left until they expire.