Activating Office unlocks the full desktop apps and removes the trial/unlicensed watermark. Here's how to do it after receiving your product key.
If Office isn't installed yet
Go to office.com and sign in with (or create) a free Microsoft account.
Select Enter a product key and type your 25-character key exactly as provided.
Follow the prompts to download and install Office, then sign in with the same account when prompted.
If Office is already installed
Open any Office app, such as Word, then go to File → Account.
Select Change Product Key and enter your key.
Office validates the key with Microsoft's servers and activates within a few seconds.
Confirm activation
Go back to File → Account — Product Information should show "Product Activated" instead of an unlicensed warning.
