A Maintainer is someone who maintians a ?Package.
The maintainer of a package is identified by having
their name and email address in the
of the ?control file, e.g.
Maintainer: Main T. Ainer <email@example.com>
A package may also be maintained by a ?team. In that case it has more than one maintianer and the ?Uploaders field in the ?control file is used to identify them, with the maintainer field either identifying the primary maintainer, or a mailing list used by the team to coordinate their work.
Sometimes a maintainer may be ?MIA, or neglectful of the package, and so it may have to be ?adopted, or ?hijacked. This is done by the new maintainer making an upload with their details in the maintainer field.
There can be different people named in the maintainer field in different versions of the package, notably in the version in ?stable, and the version in unstable. It is the person named in the version in unstable that is the maintainer, as if the maintainer changes then the version in stable is not updated to reflect that.