AndroidAPS is an artificial pancreas. They are not legal yet, or there are a few commercial ones with inferior features compared to this software and which are very expensive. The software reads your glucose from your upper arm using a sensor, and either gives you insulin or stops the delivery based on the estimate.
It connects to many different glucose sensors and pumps, provides a common interface and algorithms for them.
It is the research platform for commercial software to come. It is extremely helpful for Type 1 diabetics. Because it does not have approval as a medical software you cannot distribute binaries. You can legally compile it from sources, which is why it is for Android where you can easily sideload apps compared to the competing platforms.
xDrip is a tool that connects many different glucose sensors and gives you real time info about your levels. It enables continuous measurements with systems such as Freestyle Libre, which was only working with NFC officially.
The open source diabetes scene is quite popular in Germany. It has made our lives with T1 much easier.