Auto-enrollment upon course completion is a function that lets the system automatically enroll a learner in a new course when another course is completed.
Go to the course you want your learners to complete in order to be enrolled in another.
Under "Next course", click "Add next course". Choose the course your learners are to be enrolled in.
Create an automated communication in the next course to let the system automatically send a welcome email to all registered participants (not mandatory).