It means you must have a downloadable version of your game that is specifically marked for and runs on Linux.
Primarily, the jam was created for Linux users to make and share games, but developers on other platforms who want to explore and gain experience supporting Linux are very welcome too.
"Build" is a very established term in software development for the software artefacts that are the output of development - the bits that users end up running. So you also might see terms like "development build" or "release build" in addition to builds that run on specific platforms or in specific environments.
