If after high school you choose to live in your parents' house, keep the same job, and spend time with the same friends, your mother might worry that you've decided to stagnate, or stop moving forward.
When water stagnates, it stops moving and then sometimes begins to smell bad and attract mosquitoes. Think of a marshy pond or an old puddle. Similarly in the figurative sense, when the economy or society stagnates, it doesn't just sit still, it loses momentum. The word carries with it a negative connotation as something that stagnates really should be moving.