The changelog is here: Fairphone 2 - FP2 - LineageOS 14.1 Changelog
Amongst the changes for the 20180123 build is this: Bump security patch level to 2018-01-05 (links to the code review).
If you want to know what exactly the security patch level includes, you have to head over to Google’s Android Security Bulletin: Android Security Bulletin—January 2018 | Android Open Source Project