The purpose of this feature is to prevent the misuse of tickets for paid events, i.e. forwarding the ticket code to others.
To turn the feature on:
Move to the "CONTROLS" module > "Premium features" > "Single session restriction".
It works on the principle that if participant A enters the event via his/her email address and at the same time participant B enters with the same email (could also be the same person on another device/browser), participant A will receive an error message and his/her session will be terminated.