TE = Training Effect. It's not a Peaks thing, it's Firstbeat, a research company Garmin (and Polar and Suunto) buy algorithms from. It uses heart info to determine how much stress a workout put on your body and what effect it had on your fitness. Here's their white paper.

It's only one stat among many but generally tracks pretty well against RPE, including yesterday. Earlier this week I got TE = 4.2 and IF = 1.205 for about half an hour of hill repeats day.