A few conventions move from site to site every year, and prospective concoms bid for the right to host them. The members of these peripatetic conventions vote on where the future con will be held one or two years in advance, choosing from a slate of contenders: the bids.
A bid normally consists of a bid committee and a proposed site. (But see 7 in '77 for an exception.) To win the opportunity to host a Worldcon, a group of fans would organize a bid committee and start campaigning two or three years before the site selection vote. For a Worldcon to be held in 2020, for example, the vote would be held in 2018, so bidders would typically announce their intentions around 2016 (although some bids have started years early, and the time period involved has changed over the years).
There are rarely more than one or two bidders, but even if there is only one, an active bid campaign is essential because the goodwill gained by a well-run bid translates into an eagerness to attend the resulting convention and — especially with Worldcons and their high fixed costs — a few more members translates disproportionately into more money to put into niceties at the con.
Bids pay their campaign expenses and build support, in part, through sales of pre-supporting memberships, although volunteers on the bidcom usually pay their own travel expenses. Recognizing that these funds go to pay for the parties, many fans will pre-support more than one bid.
Conventions that move around include Worldcon, Westercon, Eastercon, DeepSouthCon, Smofcon and Corflu. The larger the convention the more elaborate the bidding process. See Worldcon Bids for more on Worldcon bidding and Other Bids for, well, other bids.