Redlines are an ion Pro feature. They are useful for dynamics environments where things don't go right 100% of the time and/or if your organization is continuously improving rather than planning everything up front.
Redlines allow engineers to change work instructions for Runs that are in-process. To redline a step,
Open the Run summary view
Find the step you want to redline
Click the three dot menu (⋮) on the step
Edit the step content in the editor
Submit the redline
You can the view the changes introduced by every redline by clicking the Show Redlines button on the redlined step.