Mappings files are used to map the patches to Lunar Client's source code. Mappings needs to be updated each Lunar Client update. Those mappings are .stm files. (STM stands for Solar Tweaks Mappings).
If Solar Tweaks told you to update your mappings, you can do so by downloading the latest stable mappings file here.
Once you dowloaded the file, you can open up Solar Tweaks and click on the Settings button (Which is marked with a gear next to the Discord button).
When the settings menu is open, click on the green button called Import from file like on the image below. And then choose the file you just downloaded and press Choose.
