>>/19048/
First lactate threshold, aerobic threshold, whatever you want to call it. Basically the infliction point where your blood lactate levels start rising. If your LT1 is high, then higher power is still just z2 for you, ie. you don't get as fatigued and you can probably produce better power later on. 
For me my FTP has risen a lot during the years but my LT1 hasn't as much, at the beginning of this year even 240W started to feel like tempo after a while. We have been working on it with my coach and it appears my LT1 has finally risen quite a bit to something like 270-280W.