Hi Stefan,
Thanks for your question. You can create notifications linked to individual activities. These notifications are based on what's called a trigger event - in the case of an activity-level notification, that trigger would be the user completing the activity. You can then decide to send one or more notifications following that trigger, either when the trigger event occurs or a set time afterwards. So for example, you could set up a series of microlearning emails which are sent 1 month, 2 months, 3 months after the user completed a course or activity.
If you look at the Notifications course Matt mentioned, you'll see a video on Creating and editing a notification which shows how to do that at a site level. And in our Courses and categories course you can see a video on Setting up course and activity notifications at the individual course level. You can also find more information in our Help documentation.
If you complete the Notifications course, you'll also receive some spaced notifications in the weeks following the course to experience this yourself.
I think these notifications are really powerful, and we're starting to experiment with how we can use them even more in the Community. Would love to know how you get on with using them :)