It usually refers to 'activating your Windows license'. Typically, a pop-up appears prompting you to do this within a set period of time - failure to comply will block a number of useful features, e.g. personalization. Also, there will be a rather visible watermark on your desktop area.
This is not the same as 'registering your laptop' with the laptop manufacturer. Registration is optional and has no impact on the laptop itself.
Cheers,
GreyCatz.