Warning: Don't flash a new ROM to your device until you are 100% sure you know what you are doing.
This custom ROM has built-in microG free-as-in-freedom re-implementation of Google's proprietary Android user space apps and libraries.
Why should I use microG instead of the standard Google Play Services?
First of all, freedom. I can't explain here why free software is so important (this is a FAQ, not a 200 pages manual), but if you're interested you can start by reading this.
Second, the Play Services are very expensive in terms of resources, they drain lots of battery and they use lots of space, while microG requires much less resources. To give you an idea, the smallest possible package of OpenGApps ("Pico Package") on ARM 7.1 is ~125 MB, while the full microG suite (GmsCore, GsfProxy, FakeStore, MozillaNlpBackend and NominatimNlpBackend) is ~4 MB... Read the Full microG FAQ