The Community user will bump non-negatively scored questions that have at least one answer scoring 0 and none scoring more than that.
Questions are picked randomly from the top-viewed inactive questions.
The number of unanswered questions bumped per hour depends on the site: 4 per hour on Stack Overflow, 1 per hour on Meta, and 2 per hour on Super User and Server Fault. The default for a new site is 1 per hour.