An activation code is a special unique registering code you need to insert during the sign up procedure in order to validate your identity.
Validation of your identity at Wi-Fi hotspots is required by the Directive of European Parliament "2002/21/EC" which, among other issues, regulates access to public computer networks. Any public Wi-Fi Hotspot in any European country is mandated to obey to this directive.
You can get an activation code at any Trampoline Point listed here. After showing your ID or Passport, the people at Trampoline Point will give you your unique activation code. Please, insert your activation when required during the sign up procedure.