Nightscout (also known as "CGM in the Cloud") is an open-source cloud application used by people with diabetes to visualize, store and share the data from their Continuous Glucose Monitoring sensors in real-time. Once setup, Nightscout acts as a central repository of blood glucose and insulin dosing/treatment data for a single person, allowing you to view the CGM graph and treatment data anywhere using just a web browser connected to the internet.
There are several parts to this system. You need somewhere online to store/process/visualise this data (a Nightscout Site), something to upload CGM data to your Nightscout (a Master/Uploader), and then optionally you can use other devices to access/view this data (a Follower or usually several):