Side project
Software a developer builds outside work, on their own time, because they wanted it to exist.
What it tells you about a developer
A side project is the clearest evidence available of what a developer finds interesting when nobody is directing them, and it shows taste, curiosity, and finishing habits that no resume line carries. Where there is none, the work history carries the read on its own.
A side project has no manager, no deadline, and no paycheck behind it. That is what makes it informative: the developer chose the problem, the tools, and how far to take it. Side projects run from a fifty-line script to a tool thousands of people use. Many strong developers have none, because their free hours went to a family, a different craft, or rest.