The movie shows a zoom into the Galactic Center and its central black hole. It starts with an image of the "Minispiral" - hot gas which rotates around the Galactic Center and that can be seen at radio wavelengths. When we zoom in, the white dot showing up in the middle is the radio emission from the direct vicinity of the Black Hole as we see it with high-resolution radio imaging (VLBI). A computer simulation sharpens the image even further showing the very bright relativistically distorted surroundings and the "shadow" of the black hole (the dark region in the middle). This shadow is caused by the "event horizon" - the boundary surrounding the black hole no photon can escape. Such an image of the event horizon could be possible already within a few years thanks to the rapid development of radioastronomy.
You can get further informations on an advanced level about the Galactic Center and its black hole in an electronic Slide Show.