Its a percent OF busy time, so without the data up above it is totally meaningless other than a ranking of most busy devices/apps to least. All of the % if you looked at every app in the list should add up to 100%.
If you take the total busy time from up above (cut off in your shot), lets say it is 1 hr for example. Then you take the % of busy * that total busy time you get 21.4 minutes out of that 1 hr for your top app. So that means 21% of the time the hub was not idle it was handling that device/app.
So by itself that means pretty much nothing. If the hub uptime is 24hrs and the total busy time is 1hr then the hub was idle for 23 of those hours. So in that case 21 mins / 1440 mins of uptime that top app only used 1.45% of cpu time for the total uptime.