Basically, Wi-Fi Sense lets people log in to your Wi-Fi network without needing the encryption password (you did set up encryption, right?). OK, that sounds dangerous and scary, but it isn't quite as bad as you might be thinking.
The same goes for a business Wi-Fi network. It might ask you to provide your name, email address or phone number before you can log in. Wi-Fi Sense will provide that information for you automatically, so you can just start using the network.
If you choose to share a Wi-Fi network, Microsoft will store your encryption password on its servers. When a visitor that has Wi-Fi Sense enabled shows up, Windows will log them into your network automatically. Your visitor will never see the password, however.
The good thing is that to get on your network, the person has to be a contact of yours in Outlook (or on Outlook.com), Skype or Facebook (with an add-in). So, if your parent or child shows up, they get logged in. A random stranger walking by your house won't.
One concern, however, is that a near-stranger could get logged in. There are some friends you have on Facebook that you might not actually want on your network. Or maybe a long time ago you created a contact in Outlook for someone you emailed with only once or twice.
True, if you don't know the person that well, what are they doing near your house? Unless it's that neighbor you got in an email war with over where to put the trash cans on the curb. Do you want them on your network?
Still, if you don't want someone on your network, you can remove them from your Outlook, Skype or Facebook contacts. However, if you actually need to contact them, then that's not always practical.
Wi-Fi Sense does require that you voluntarily share your Wi-Fi network. If you don't share a network, then no one gets logged in. But that still lets you use the part of Wi-Fi Sense that logs you into other places automatically.
If you'd rather shut down Wi-Fi Sense completely, you can do that, too. Windows 10 gives you the option during setup to share your Wi-Fi networks with others, or share your credentials with other networks. Just tell it no to both. You can also find these options in the Wi-Fi Settings screen.
If you want to be really sure that Wi-Fi Sense stays completely away from your network, you can change your network name in your router to include "_optout" (minus the quotes) at the end. So, if your network name was "my network" you would make it "my network_optout". Learn how to change your network name, and pick a good one.