Required or sticky steps allow you to ensure that certain important steps in a workflow aren't skipped over.
By default, steps in a workflow are specific to that stage. When a lead or a process is moved out of a stage, any outstanding steps left in that workflow are then removed, and the lead/process get started on the new workflow associated with their new stage.
However, using the required, or sticky step, feature, you can ensure that a step will follow the process into other stages, until that step is completed. It won't be removed by a stage change, but will stick to the process.
To set a step as required, go to the settings page for the process type and click into the stage you want to edit.
On the step you are wanting to set as required, click the star icon on the right of the step. When the icon is blue, this is now a sticky step.