The Mathom-House it was called; for anything that Hobbits had no immediate use for, but were unwilling to throw away, they called a mathom. Their dwellings were apt to become rather crowded with mathoms, and many of the presents that passed from hand to hand were of that sort.
--J. R. R. Tolkien, in the Prologue to The Fellowship of the Ring
In short, it works like this--bring your mathoms to the Mathom Pile. If somebody else wants it, they can take it for their own. If you find something that you want, you can have it.
Please note that there is a difference between clutter and a mathom. Clutter is stuff you're hanging on to because you haven't had a chance to take it to the thrift shop. A mathom is something you're hanging on to because you haven't gotten around to selling it on eBay because It Might Be Worth Something.
If you subscribe to the KonMari method, a mathom is something that no longer sparks joy, but that you feel would spark joy for somebody else.
If you do have a stack of Stuff to Get Rid Of, here's how to sort out the clutter from the mathoms before you come over:
Anything in the Mathom Pile that is not claimed by midnight will be disposed of by the hostess. (So make sure you really wanted to get rid of it.) Thank you and goodnight.