The "founder" of a wiki is the person who first created the wiki. Founders do not have any special authorities within the wiki and they do not own the wiki that they founded. They simply created it first.
Founders are automatically given administrator and bureaucrat access to their wiki, but the purpose of this is to allow them to help get the wiki started through customization, not to give them control over the content. Any other regular editor of the wiki can also become an admin or bureaucrat. This can happen either by asking the founder and/or the community of the wiki or by adopting the wiki if the founder has since become inactive.
Determining a wiki's founder
One way to determine who the founder is (and whether they are active) is to visit Special:ListUsers and search for "Administrators" and "Bureaucrats". Oftentimes, admins with both user rights are the founder. On larger wikis you may see a few users with both rights. This does not mean they are the founder, as only one user can found a wiki.
The best way to find out who the actual founder of a wiki is to check each admins page. The founder of the wiki will have the founder badge on their profile.