Upstream is the term used to talk about the developers of the software that you are packaging.
As a package maintainer you will normally be asked to talk to upstream about things such as bug reports and new versions.
If there is no upstream development for your package then you will in effect be upstream yourself, as you will have to deal with any bug reports on your package. There might be some help on the unmaintained free software wiki though.
The upstream developers may also ask you to be a contact with bug reporters, to get more information on a problem.