A "maintenance programmer" is a programmer whose job is maintaining a system (bug fixes and adaptation to relatively minor environmental changes) that is viewed as complete and basically static, either without major new development expected or with major new development done by separate teams in as-needed projects. Often, maintenance programmers are employed by the organization owning the system, while new development is done by contracted teams.

It is a particularly common role in government and other enterprise shops, particularly those that still use a pre-Agile, civil-engineering-metaphor approach to software development.

As I understand the term it's someone who inherits the feature-complete codebase after the original author left it. The duty of such a programmer varies depending on the place and project, but it's generally someone who will be asked to fix the bugs and implement new features should the need arise.

There's nothing derogatory to the term, although it's good to have some sympathy for such people - enough not to write bad code when first coding the project in the first place.