Glossary: Level
"Leveling the project", or "leveling resources", means automatically rearranging the project to make sure the resources are being used as efficiently as possible. This means not allowing a resource to be assigned at more than 100% of its available units at any time, and finding the best order for resources to work on tasks in order to complete them more quickly.
Newly created projects are set to automatically level resources as they are allocated to tasks. You can disable automatic leveling in the Project menu.
When not leveling automatically, you should level your project after making changes like updating task completion or changing resource assignments.