Imagine this scenario: a friend visits your house, and wishes to use your
Wi-Fi.
You tell your friend your Wi-Fi password in confidence. They type it into
their laptop, which has recently been upgraded to Windows 10. Your friend's
laptop can now use your Wi-Fi connection, and you're fine with that. After
all, they're your friend. You don't believe they are going to abuse the
privilege. If you've chosen a complex, lengthy password like 7sAp
{oMEmGD#YT)MzGdTLQM9C then chances are that your friend is not going to be
able to remember your Wi-Fi password, even if they wanted to break your
confidence. So far, so good.

But there's some bad news.
And that bad news is that your friend has Windows 10 installed on their
laptop. And Windows 10 has a password-sharing feature called Wi-Fi Sense
enabled by default. Requiring just a click and the password being re-entered
by your friend, Wi-Fi Sense allows your friend's Skype and Outlook/Hotmail
contacts (if they use Windows 10 or Windows 10 Mobile) to also log onto your
Wi-Fi network, without you having to share your password with them. Access
can also be shared with your friend's Facebook friends if they choose to
allow that.

In other words, complete strangers could now use your Wi-Fi without your
permission. And the password is now stored on Microsoft's server.

Microsoft's answer? If you don't want Wi-Fi Sense to scoop up your password
when you share it with a friend, and then share it with that friend's
friends and contacts you have to rename your Wi-Fi network.