I have struggled with low blood pressure for most of my life.  Sometimes nurses are shocked when they see what my blood pressure is.  Basically, it means that your blood is not pumping at the rate it should.  Most people worry about high blood pressure, which can cause heart attacks, but low blood pressure is dangerous too.  It can make you weak.  Since I am not a doctor, I sought out an article for you.

http://www.enotes.com/blood-pressure-reference/blood-pressure

http://www.enotes.com/hypotension-reference/hypotension (low blood pressure)