The most basic definition of the Anaerobic Threshold is where the body generates more lactic acid than it can flush out. So if you were to make a line graph in which one line represents the rate lactic acid increase and the other line represents the rate of lactic acid decrease, the point at which the lines cross would technically be the "anaerobic threshold". Lactic acid builds up faster as the intensity of a workout increases. The rate at which it's disposed of is relatively constant.
Sprinters can train above this point without much issue because their running is in very short bursts. Endurance athletes though need to stay below this point in order to continue on without having to stop for rest. You can, however, improve your ability to remove lactic acid through consistent training. As such, the speeds, distances, and times would increase.
I think the "metabolic shift" part comes from where people are told that at a certain intensity, your body starts generating energy through anaerobic (no oxygen) means rather than aerobic (oxygen). When you're aerobic, you generate energy by using fat whereas you would use glucose if you're in the anaerobic phase. The idea being that you would burn more fat if you stayed within this aerobic center. You'll often see charts on cardio machines that say "aerobic between 60% to 70% of VO2max" or something.
This is a general over-simplification of how the body generates energy. What the body chooses as a source of energy is far, far more complex than that. It is based off a large number of factors. Also, the body is using both systems for energy pretty much all the time. You are in fact using both energy systems right now reading this while at rest (I assume you're at rest).
The body will start using more of its anaerobic energy system as a workout increases in intensity because it needs that extra energy to perform its duties. It will, however, never stop using the aerobic system.