Mercy regenerates health, but only when she has been out-of-combat for a couple of seconds. The only other concept of regenerating health in the game is shield, which also seems to regenerate out-of-combat. Is Mercy's regen any different from shield? Or does Mercy practically just have 200 points of shield health?
Mercy's out of combat (more accurately, 'not being shot for 3 seconds') HP regen is significantly slower than shields. The regen does effectively mimic that property of shields, but at a slower rate (appears to be about 10hp/s for Mercy, vs 25/s for shields).
Notably, shields only regenerate themselves, and not the health under them - so if Zarya, with 200 health and 200 shields, takes 300 damage, her shields will only bring her back to 300 combined health (100 health and 200 shields). Mercy, on the other hand, seems to regain her entire health pool.