A "founder" is someone who created a community.
Founders are automatically given administrator and bureaucrat rights on their community, but do not have special privileges or abilities beyond this. They do not "own" the community that they founded, but can set the early direction and design of the wiki, along with choosing who joins the local admin team.
Any other regular editor can also become an administrator or bureaucrat. This can happen either by asking the founder, another bureaucrat, the user base of the community (if permitted on an active community) or by adopting it if the founder has since become inactive or if there are zero active bureaucrats and admins on the community.
Determining a community's founder
One way to determine who the founder is (and whether they are active) is to visit Special:ListAdmins. Often, admins with both Administrator and Bureaucrat user rights are the founder. However, on some communities, you may see a few users with both rights.
The best way to find the actual founder of a community is to check each admin's page. The founder will have the founder tag on their profile.