One of the postulates of A. Einstein's special theory of
relativity, which puts forth that the speed of light in vacuum is measured as the same
speed to all observers, regardless of their relative motion. That
is, if I'm travelling at 0.9 c away from you, and fire a beam of
light in that direction, both you and I will independently measure
the speed of that beam as c.
One of the results of this postulate (one of the predictions
of special relativity) is that no massive particle can be
accelerated to (or beyond) lightspeed, and thus the speed of light
also represents the ultimate cosmic speed limit. Only massless
particles (collectively called luxons, including photons,
gravitons, and possibly neutrinos, should they prove to indeed be
massless) travel at lightspeed, and all other particles must
travel at slower speeds.
See tachyons, causality principle.