The LTHR is the point, above which, increased blood acidification occurs in your body. Crossing this threshold will make your endurance performance rapidly decrease. In a race, you might maintain that level for up to an hour, but at a certain point you will slow down. When the lactate clears from your system you can pick up the pace again
If you were to do a sustained, maximal, one-hour effort your average heart rate during that effort is your LTHR.