The theory of relativity predicts that a particle can never be accelerated to a speed faster than the speed of light, but physicists have long known that (in theory, at least) it would be possible for particles to move faster than the speed of light, as long as they don't have to accelerate to get there. Tachyons, which always move this fast, are sometimes hypothesized in physics theories to serve some useful purposes.
Though tachyons are sometimes referenced in physics, the overwhelming majority of physicists today recognize the appearance of a tachyon in physics as representing a fundamental instability in the theory being worked with, and there are mathematical tools developed for dealing with these instabilities so that the theories can work. No evidence has ever been found that would support the existence of actual tachyon particles.
Just to be absolutely clear: Most modern physicists do not believe that tachyons actually exist. There's currently no evidence that any physical object moves faster than the speed of light.
Despite this, tachyons show up in science fiction a lot, often related to some form of time travel, since their existence would cause potential violations of causality and allow for information (including matter) to be sent backward in time.
Also, some holistic healing practitioners have latched onto it as a means of improving human health. There is no real physics evidence to support this claim since, as mentioned above, physicists don't really believe that tachyons exist.