Wi-Fi Sense is apparently a feature that first appeared on Windows Phone 8.1, and is described by Managing Editor Sam Sabri in this Windows Central article from last year — without irony or sarcasm — as a, "killer feature." The apparent use case for this "killer feature" is to more conveniently share the connection credentials to your own WAP with your friends. If, however, you would prefer your WAP's info to not be shared, you have but to append the string "_optout" to your SSID (no solution is provided for people whose SSIDs are already near the 32-character limit). The WinPhone version of Wi-Fi Sense reportedly does not display the WAP's password to recipients but, since recipients can connect, the password is (probably) stored using a symmetric cipher and, thus, can be easily extracted. Wi-Fi Sense will also automagically click through any ToS page that typically appears on public WiFi access points (thus destroying any remaining illusion of meaningful assent to such so-called contracts).
Wi-Fi Sense can apparently be turned off completely, but its default state appears to be enabled and sharing everything. It is unclear how much, if any, of this "killer feature" will be in the final release of Windows 10.