Also known as a critical power curve, this feature (only available to power meter users with a Strava subscription) provides a picture of the maximal power you have sustained for a given time interval (horizontal-axis).
The Power Curve shows your best average power for time periods of 1 second up to the length of your ride. We search your entire ride and find these best efforts, and you can compare them with your best efforts for your last six weeks, the current year, or years in the past.
The Power Curve can be displayed in Watts (W) or Watts per Kilogram (W/kg.)
To view your Power Curve, hover over the Training tab at the top of any Strava page, then select Power Curve from the drop-down menu - or visit http://www.strava.com/athlete/analysis.
From this page, you can view and interact with your Power Curve. To make comparing different time periods easier, you can plot two different time periods separately on the same chart - and customize the date range for each one.
Clicking to show Estimated FTP will also display a dotted line to represent Strava's estimation of your FTP with regard to each time period you have selected. You also can toggle between displaying Watts and Watts per Kilogram (assuming you have entered your body weight on your Profile Settings page).
Any power goals you have set will appear as small orange circles at the relevant spot on your power curve.
Clicking on any point in the graph will 'focus' on that time interval - and show a link to the activity during which your peak power for each selected time range was achieved. Simply click again anywhere in the chart to 'unfocus.'
Finally, for more information on how to make use of the data displayed at your Best Efforts Power Curve, click the 'What's This?' link near the top of the page; or visit this knowledge base article.