Mercy's Caduceus Staff
ability allows her to heal her teammates at a rate of 50 health/second using a yellow beam from the staff.
This achievement comes down to you surviving for long enough get enough health restored. Just use your staff on your teammates as they get injured to heal them. If you're able to heal all 5 members of your team (not including yourself) for a total of 200 health each, the achievement will unlock.
You can check the bottom of your personal scoreboard at any time to see how much healing you've done total in the match.
If you'd like to learn more about Mercy, you can view this video. The achievement is described and shown at 9:08: