Stagnant means "not moving" or "standing still". If referring to the planets in astrology this refers to a period when the planet stops in the sky, as viewed from earth, before going retrograde or resuming direct motion.
Mean can mean you are being rude or in math terms it is adding a set of numbers then dividing by the total amount. ex. If your numbers are 1,2,and 3 add them to get 6 and divide by 3. You get 2.