A founder is a role granted automatically to users who create a wiki.
Beyond that, founders receive no special privileges or treatment and they do not "own" the wiki or community as per the wiki philosophy (and licensing). Founders are intended to help guide the wiki in its early stages, set its direction, establish its design, help nurture a community and eventually grant admin and even bureaucrat rights to other users.
If a founder is inactive and a wiki does not have additional bureaucrats, active users may consider adopting it to receive these rights and continue the founder's work.
Determining a community's founder
The easiest way to identify the administrator is to check Special:ListAdmins. Founders will usually have both Administrator and Bureaucrat roles. The founder will always be identified by the founder tag appended to their profile. Alternatively, you can check the page history for the main page, where they will usually be one of the earliest contributors.