What is a trade script? And why i would need it?

Hi all,
i've searched the forum adn bigG but still cannot find a exahustive answer...

What is a trade script? And why i would need it?
I've read about it in about all adult CMS (tube scripts etc), i've seen features of the most popular script but still don't understand what's the roundtrip?
I know only that trade make me think some sort of exchange (links i think) but how it happens? It's like something to automate ad-space selling?

Thanks much.

PS. If there is already a topic about it, please write the link here, i'll read it all!