UFO = Unidentified Flying / Floating Object. Do people ever see objects flying in the sky that they can't identify? Yes. Therefore UFO's obviously do exist. What does it mean? It means that we are not always able to identify flying objects.
I think that credible investigations have revealed that the vast majority of UFO sightings are caused by errors of perception, where very innocuous objects are mistaken for alien spacecraft because of poor viewing conditions or other peculiar circumstances.
In some cases the UFOs are actually experimental military aircraft being test-flown. It may also be satellites. A distant aircraft can appear to accelerate unnaturally fast if it changes direction from moving toward the observer at a slight angle, to moving perpendicular to the observer's line of sight. A large number of claimed UFO footage are also hoaxes. A lot of the documentaries on UFOs are sensational in nature, designed to appeal to conspiracy-minded viewers rather than actually giving the objective truth.
There is a lot of fakery, half-truths, distortions, exaggerations and wild imaginations when it comes to UFO claims. There simply is no real evidence of extraterrestrial visitations, and I'm saying this as someone who thinks it very likely that there is life out there somewhere in our vast universe. But knowing the vast size of our universe and the tremendous distance between stars I think it very unlikely that alien life would be able to physically travel to us and back. The vastness of space is virtually an impenetrable barrier to interstellar commuting between planets that host intelligent life. I think the likelihood of there being life out there is greater than the likelihood of them being able to come to us or we go to them. The most we can hope for is picking up some form of extraterrestrial communication broadcast - assuming they're even technologically advanced enough to achieve such a feet. And that's another thing. We seem to always assume that alien life would be just as advanced or more advanced than we are. Maybe the majority are primitive superstitious beings with very little or no scientific or technological knowledge. The universe could be teeming with intelligent life, but we could be one of very few or perhaps the only ones who have advanced technologically to the point of being able to broadcast to outer space.