oil pressure is the pressure of the oil as it moves through an oil passage in the block. if you have a worn part that makes that passage bigger the pressure will drop. main bearings have a hole where the oil is applied to them. however lots of things can cause low oil pressure such as a bad oil filter.
http://www.2carpros.com/articles/high-or-low-engine-oil-pressure - a little info
http://www.aa1car.com/library/us1097.htm - alot of info
