Answer:
UFO means "unidentified flying object", which means you look up and see something and don't know what it is. In that sense UFOs are real. There are plenty of natural and man-made thing in the sky that are hard to identify.
Some people think "UFO" stands for "Alien Spacecraft", which would of course be "A.S.". In real life, neither aliens nor alien spacecraft are known to actually exist, which makes them the least likely explanation for these events.