That's Bitlocker. To get the key, type "Bitlocker" into Search, then go to "Manage Bitlocker" in the Control Panel. There will be an option to back up your key, which you can use to put it in a file that you should then be able to use to unlock the other partition.
On that note, I'll add that since I set up my SP3 for dual-booting Win 10, my Bitlocker shows encrypted in modern settings, but as turned on in Control Panel. So, I think partitioning for the dual boot confused Bitlocker somehow. Would probably need to restore to factory defaults to get it back, so I better not leave this thing anywhere, because it's definitely off and I can access my other partition from this one...
If someone can login to your computer, BitLocker serves no purpose. It is only useful if someone removes the SSD drive and tries to access it or if the computer is tampered with it could request the key. I don't think people realize that the computer is setup with BitLocker on and everything they add to the SSD drive is encrypted. If they don't backup the BitLocker key and the computer ever changes to the point it asks for that key, if you don't have it, the computer is bricked.