The Web Portal is the interface in your browser: you sign in with your parent account and change time limits and rules from any computer – without picking up your child's device.
This video walks through the interface once and shows where the settings and the reports are. If you look after several devices in the family, this is the single place where you manage them.
The written version is the web portal guide. To sign in directly, go to login; the other films are in the video overview. How time can be counted across several devices at once is explained in bonus time and roaming.