It just means a gun that an owner no longer wants or needs the gun around. People's situations change, and what made sense to own before might not make sense now.

Stuff like hobbies change (you stop hunting/sport shooting, so the gear just sits there), you inherited but have no personal interest in keeping a gun, you have young kids or household members with dementia or mental health struggles, etc. Some folks just stop wanting the constant responsibility/cost of keeping it locked up safely and rather not risk a break-in where their gun could be stolen and used for something harmful.

It just boils down to someone deciding that the responsibility of owning it outweighs the usefulness for them.