NMU is defined in the developer’s reference as:
In some situations, it is useful that other developers can upload a new version as well, for example if they want to fix a bug in a package they don’t maintain, when the maintainer needs help to respond to issues. Such uploads are called Non-Maintainer Uploads (NMU).
The idea is to fix bugs even if the maintainer is away or busy. There’s a different process to deal with non-reactive maintainers.