After you activate the software product, a specific product key is assigned to the computer hardware on which you installed the product. After this product key is assigned, you cannot use the same product key to activate the product on other computers.
You can re-install Windows on the same computer as many times as you want because activation pairs the Windows product key with information about your computer hardware. If you make a significant hardware change, you might have to activate Windows again.

You can read more about activation on this Microsoft Activation article.

Hope this helps.
Please reply if you have any further questions.

Thanks and Regards,
Manpreet C