When you don't use or move something for a long time, it starts to get a lot of dust on it. So things that don't get used are said to be "gathering dust". Examples of things that might "gather dust" in your home are:
- unread books
- strange kitchen appliances
- rackets, weight sets, and other sporting equipment
There's also a similar phrase, "collecting dust":
It's just collecting dust in a closet somewhere.