A self-effacing person does not want eyes on them or on their successes. They will pass the praise along, stay at the edge of the photo, and talk about “we” when “I” would be fair. It is an adjective, a little formal, used when you want to praise someone’s quietness without calling them shy. Shy is about feeling tight and afraid; self-effacing is about not claiming space. Writers like the word because it is more precise than “nice” or “humble,” which can mean many things. Though actually, people also use it with a hint of worry: a self-effacing colleague may be easy to overlook, and the quietness can hide skill.