Achievements are automatically generated by your workout data. Achievements include your best times (personal records), distances swum, and movement on leaderboards. If you achieved a new personal record or passed someone on a club or pool leaderboard, they will be noted on the individual workout in which you reached that achievement (first screenshot below) and also on your main achievements page (second screenshot below). As more workouts are added by both you and your connections, certain achievements will change. For example if on your next workout you set a new personal best for your fastest 50 meters, that achievement would disappear from the old workout and appear on your new one.