When a lesson opens according to the schedule, the student receives an email informing them that a new lesson in the course is available, with an "Open" button.
Emails are sent according to a schedule: the system selects lessons with a pending opening date and notifies students with access, taking into account the stream, if one is set. For this to work, the lesson opening notification setting must be enabled in the course. The email subject should be about the opening of access to the lesson.
Additional chains around access and start are configured in automatic actions—see How to automate the start of training?