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.