It's a collection of your best power efforts for various durations plotted against time. IOW, a continuous graph of your best efforts from 1 seconds out to the longest ride you've done with a power meter. What's your best 3.5 minute power? Just look at that point on your MMP. How about your best effort for 25, or 30 minutes with a TT of roughly that time coming up? Again, just look at it on your MMP.
There are a couple of gotcha's on the MMP. It's possible for instance to have some small hills in the MMP where it shows a higher power for a longer time. That doesn't make sense in a physiological sense, but it can happen if for instance you do two hard power spikes say nearly 1 minute apart. You can end up showing lower power for 45 or 50 seconds but those two spikes contribute to a higher 1 minute average power. Don't take these sort of data anomolies too seriously, they're not real important in the big picture.
The MMP will also only show what you've actually done, not what you're capable of. So if you don't actually go out and do a best effort hour long time trial you won't actually show your FTP on your MMP. Same idea but even more so for two and three hour or longer efforts. It's likely you're capable of better long MMP numbers than the graph actually shows unless you've done some focused best efforts for those very long durations. So the MMP doesn't try to predict what you're capable of, it just records the best efforts vs time that you've actually done to date.
for any given duration, the best average power you achieved during the time frame chosen (this year, all dates, last 28 days, etc...). ie. your best average power over 2", 10", 1', 30', 2 hr duations and everything in between (cherry-picked from all your data files). plotted on a linear/log scale. so, if you wanted to see what your best ever average power is for a 45 minute effort, you look at the X axis for 45 and see what your power is on the Y -axis. you can also mouse over data points to see the date and data for specific highpoints.
a very cool function is to have 2 curves on the same page- representing 2 different date ranges- for example, all workout vs last 28 dates. or this season's vs last season's best.
basically what you are looking at is the highest power (on the y-axis) that you have achieved for each duration... 1 sec, 2 sec... 10min.. 20min... 1hr etc (on the x-axis) for whatever time period you define (like "this season etc.)