This is an interesting case and both sides are putting forward some good arguments for and against. The crux of the matter is that Google forces manufactors that wish to bundle their market leading apps such as Google Search and GMail etc to include a closed component of the Android Operating System (OS) called Google Play Services. The commission argues that this give Google an unfair advantage and allows it to impose discriminatory practices on OEM as well as other application developers.
On the flip side Google argues that this is essential to ensure compatibility and security. Furthermore, it says that is requires fewer apps from Google to be bundled versus the other leading competition which is Apple who has an entirely closed system and includes no third party apps by default. Additionally, its flexible system is open and external developers have had no problems releasing best in class applicaiton through its App store. Just take a look at Snapchat etc that beat of all Googles miserable messaging efforts in the form of Hangouts, Allo and Duo.
A fascinating case. It will be interesting to see which argument ultimately wins out in the end.