Estimated best efforts are your fastest running times for distances such as 1 mile, 5km, 10km, and half marathon. Strava automatically finds your best efforts in each running activity and keeps track of your best performances over time.
When you run one of your top three times for a particular best effort distance, the result will appear in the achievements section of your activity. You can also find a complete list of your best effort times on your profile at strava.com.