Sorry for the dumb question, but I see it requested for bigger trades. Can someone explain it to me?

It is a URL for the search of your item that you have listed in a public stash tab (either premium stash tab or centre slot of currency stash tab). There is a useful thread here on trading safely in PoE https://forums.d2jsp.org/topic.php?t=105345124&f=122

The easiest way to find your item in particular is to filter by account name, but can also just search for item name or specific mods/rolls.

Using tradelinks is important as GGG have historically banned quite actively for suspicious/lazy rwt like just directly handing over currency/items or throwing them on the floor.

I thought it was something along these lines but couldn't figure out how people could find their own listing. Thanks for the explanation, and I will check that post out. Much appreciated!