Wi-Fi Sense automaticly connects you to open networks, as well as, any network that your contacts from your email, facebook, and other social networks have connected to. Without you seeing the passcode for those private networks you gain the ability to connect to them, vise versa the networks you use will be opened to your contacts.
Here's the full description from Microsoft: "Wi‑Fi Sense connects you to Wi‑Fi networks around you. It can do these things for you to get you Internet access:
## Automatically connect you to open Wi‑Fi networks it knows about by crowdsourcing networks that other people using Windows have connected to. These are typically open Wi‑Fi hotspots you see when you're out and about.
## Automatically connect you to Wi‑Fi networks that your Facebook friends, Outlook.com contacts, or Skype contacts have shared with you after you've shared at least one network with your contacts. When you and your contacts share Wi‑Fi networks with each other, you give each other Internet access without having to tell each other your passwords. No networks are shared automatically. When you first connect to a network that you decide to share, you'll need to enter the password, and then select the Share network with my contacts check box to share that network.
The initial settings for Wi‑Fi Sense are determined by the options you chose when you first set up your PC with Windows 10 or your phone with Windows 10 Mobile. You can change your Wi‑Fi Sense settings any time by selecting Settings > Network & Internet > Wi‑Fi > Manage Wi‑Fi settings on your PC or Settings > Network & wireless > Wi‑Fi > Wi‑Fi Sense on your phone, and then changing one or both of these settings under Wi‑Fi Sense: " http://windows.microsoft.com/en-us/windows-10/wi-fi-sense-faq