The Gaia satellite is a European Space Agency mission to map out our own galaxy, the Milky Way. Over its five year mission, Gaia will measure the positions, distances, and motions of about a billion stars within the Milky Way. To do this, Gaia will look at the entire sky over and over, so that it can track the positions of these stars over time. But while Gaia is looking at the sky, it will also spot thousands of transients.
A transient is anything in the sky which appears, disappears or changes. Some of these transients will be stars exploding as supernovae, or black holes swallowing stars. The Gaia Alerts project is working to find such events in the data from Gaia, and announce them to the world in real time.