Andromeda, otherwise known as M31, is among our nearest neighbors, “only” 2.5 million light-years away. Its line-of-sight speed has been known for a long time (from Doppler measurements), but until now we didn’t know whether its actual direction of motion is more toward us or more tangential to us.
The answer: Andromeda is coming at us, and will “collide” with the Milky Way in about four billion years. I put the word “collide” in quotes because while the collision will have a dramatic effect on the appearance of both galaxies, physical encounters between stars (and the planets they contain) will be really rare, because there is so much empty space between stars. Our solar system will most likely be unaffected by the collision, and will be having greater problems at that time due to our Sun’s behavior; that’s roughly the same time it’s estimated that the Sun will expand into a red giant.