Bidding is the process of trying to garner enough support to bring a traveling convention to your site.
Many conventions travel from one location to another. The next location is chosen through a process known as Site Selection. The group trying to bring the convention to their individual location is known as a bidder. Details vary, but the process is essentially an election campaign, in which each bidder attempts to convince the Site Selection voters that their location is going to be the best for the next convention.
The bidding campaign has many aspects. It is a mixture of substantive convention building, good marketing, and (sometimes) politics.