Site Selection is typically an election process in which a new convention location decision is made.
Many conventions change locations. For example, the World Science Fiction Convention has a new location every year, and chooses a new location two years in advance. The new location is chosen by the members of the currently seated convention, so that, for example, the members of the 2010 convention selected the location of the 2012 convention.
The actual election process varies between conventions. Again to cite the Worldcon, they use a preference voting method, in which each voter rank orders their site preferences (or None of the Above, or No Preference). The preferences are counted, and the site with generally the most support wins the election.