An easier way to understand the difference is to look at it like this. Low pressure is similar to a kitchen stove. If you need to heat large quantities of food, or bring large amounts of water to a boil then you wouldn't want to use a low pressure cooker. High pressure, on the other puts out large amounts of heat. This is the type of cooker you would want for frying a turkey, or boiling large amounts of water for a crab boil.
BTU stands for British Thermal Unit. This is the amount of heat required to raise 1lb of water 1 degree F. This means nothing to most of us. You can just think of it as a measure to how powerful a burner is, with our low pressure putting out around 40,000 BTU's, while our high pressure puts out up to 160,000 BTU's.