Also referred to as Functional Threshold Power, it is regarded by most athletes as the power that can be sustained for an hour.  In Xert, Threshold Power closely matches that of FTP.  In general, for most athletes, Threshold Power is slightly above FTP.  Inspecting a power-duration curve using the Xert Fitness Signature calculator reveals the difference.  Here’s an example:

Here we have FTP = 280.8 watts and Threshold Power = 282 watts.