The ultimate expression of condescension, usually it's employed when someone is too cowardly to turn something down directly. It essentially means "No." or "I must decline.", but it sounds wishy-washy and weak. It's used so much because it's way easier than saying "No." and it also for some unknown reason sounds more polite and less blunt.
Guy: Hey do you wanna go out to the restaurant with me?
Woman: (Staring into her phone) Ummmm no thanks I'm good.
When you don't want to switch into new opportunities.
Did they offered you a new job ?
Yes, but I'm good where I'm good.