If you choose the Enroll option while having Learners selected in the Course Activity Report in the exact same Course that they are already Enrolled in, the LMS will treat it as a Re-Enrollment in the Course.
In this case each Learner's previously Active Enrollment will be moved over to the their Enrollment History at whatever state it was at the time of the move (i.e. Completed, Not Completed, etc.) and their latest, reset Re-Enrollment will become the Active one for the Course.
Related to this, should you happen to delete the Enrollments of the Learners that you select here, their Active Enrollment will be completely removed from the LMS. The information for that Enrollment will not be retained in their Enrollment History, their Enrollments Column in the Report will not include this instance, and they will be out of the Course completely.
The Learner can still Re-Enroll if the Course has been authored to allow it, otherwise they will not be able to access that training again until you intentionally allow them to do so.
The Post-Enrollments feature can be configured at the Course-level to re-enroll a Learner into a Course after a specified amount of time.