See subject. Dudes, keep it simple before having a physics free-for-all. 
My very basic layman’s understanding is that they’re essentially a type of theoretical faster than light particle. Note that this doesn’t violate the “c is the speed limit of the universe” rule, because the rule is really “nothing may CROSS the speed of light threshold, meaning FTL particles may not reach or go below c and STL particles may not reach or go above c.”
Some models of physics allow for hypothetical particles called tachyons, which have imaginary rest mass. Because of this, for their energy to be real, they must travel at speeds faster than c. Like ordinary particles, their energy increases without bound as they approach c, except that they would approach it from above, not from below.
It’s all pretty much just mathematical thumb-twiddling, though, since the same models that say that they might exist, also say that if they do, they absolutely can’t interact with ordinary matter in any way. Which is the next best thing to not existing.
It’s a gluon that’s not completely dry.
Serious answer: What Chronos and Jragon said. Note that such particles are entirely hypothetical; no experimental evidence has ever even hinted that they exist.
Basically the equations work just as well if lightspeed is the lower bound as they do if lightspeed is the upper limit, so “tachyons” are the hypothetical particles which would be described by that set of equations.
Just because they can be described and defined does not mean they exist.
Note that “tachyon” refers to a class of particles, all those that travel faster than light (have an imaginary rest mass).
Cf. “tardyon” or (“bradyon”) for regular sublight particles which includes electrons, protons, etc. And “luxons” which includes photons, gravitons, etc. (And neutrinos. I’m a bit stubborn on that one.)
Hence there is no presumption that if they exist that they will come in only one type.
Would tachyons know they are tachyons? Could there be something like a whole alternate tachyonic universe? Would its physics be isomorphic to the physics we know? Maybe WE are the tachyonic universe. Could we tell?