From the FAQ:

"""
Key
If you choose this option, you will be required to enter one or more Steam keys when creating your giveaway. Once your giveaway is complete, you'll be able to see your winner, along with their email address. You'll notice the key you previously entered is displayed next to their username, along with buttons to modify or delete the key if necessary. At that point, the key will not yet be visible to the winner. They will only be able to see the key on our site when you check the "Send Key" checkbox. If you uncheck that box at any time, they will no longer be able to see the key. An icon with an eye will appear next to the key once it has been viewed by the winner.
If you create a giveaway for 50 keys or greater, the "Send Key" checkbox for each winner will be automatically selected. This means the keys will be instantly available to your winners the moment your giveaway ends. This is done to make the experience more convenient when managing giveaways with a higher number of keys.
"""

Hope this helps!

Thank you for marking the key as received :)