For questions related to xConnect, the API service layer abstracting the storage architecture of xDB from those accessing xDB data.
xConnect is the service layer that sits in between the xDB and any trusted client, device, or interface that wants to read, write, or search xDB data. Communication must happen over HTTPS and clients must have the appropriate certificate thumbprint.
https://doc.sitecore.net/developers/xp/xconnect/xconnect-overview/index.html
https://citizensitecore.com/2017/10/17/introducing-xconnect-for-sitecore-9/
