What is Tachyon and what are benefits of that?

suppose tachyon is point star

A Tachyon is a hypothetical particle that can go faster than the speed of light. Tachyons are mass-less, whereas a superbaryon can go faster than the speed of light and has mass. If these were found, observed, and proven, they could completely throw off Relativity.

A tachyon is a hypothetical subatomic particle which moves faster than light. Since it's only hypothetical, it doesn't have any direct benefits to us apart from sparking our imagination and being an element in science fiction books and movies.

A tachyon is a theoretical particle that travels faster than light-in-a-vacuum. Contrary to popular belief, this is not forbidden by Relativity, as it is impossible for us to detect such particles nor to generate them (therefore we can't get information to travel faster than light).

The closest we have in our universe is Cherenkov radiation.

http://en.wikipedia.org/wiki/Cherenkov_radiation

It is possible to have, in some medium (for example, water) particles that will travel faster than the speed of light "in that medium" (this is not faster than the speed-of-light-in-a-vacuum, which is the speed limit under Relativity). In water, photons of light can only travel at 3/4 of the speed-of-light-in-a-vacuum. It is therefore possible for some particles to travel even faster than that.

However, these particles are not tachyons.

"Real" tachyons have never been observed. They are, so far, only a mathematical curiosity.