A maintainer is someone who maintains, develops and looks after an Open Source project. Usually with enough rights to be allowed to push changes to the necessary repositories.
In large projects, there can be numerous different roles with different people assigned to them. In smaller projects, there are often just a small set of people that do everything that needs to be done in an Open Source project. In many projects there is only one maintainer.
And projects need so much more than just code to strive and succeed.
Maintaining a project can include at least the following roles: