LineageOS is a free, community built, aftermarket firmware distribution of Android 8.1 (Oreo), which is designed to increase performance and reliability over stock Android for your device.
LineageOS is based on the Android Open Source Project with extra contributions from many people within the Android community. It can be used without any need to have any Google application installed. You will need to provide your own Google Applications package (gapps). LineageOS does still include various hardware-specific code, which is also slowly being open-sourced anyway.