What does it mean to have a blood pressure of 150/100? How serious is it?

Depends. A single elevated blood pressure reading is meaningless because blood pressure can go up for a number of reasons including pain and anxiety. Chronically elevated blood pressure needs to be addressed because it can lead to problems with the heart, kidneys an brain.
Hypertension. There is a linear relationship of the risk of stroke, heart failure, and kidney failure with blood pressure elevation over 140/90. Yes you need evaluation and treatment.