"Cee-fractional" refers to a high percentage of light speed, from the traditional use of the letter c to denote the speed of light in physics equations, such as in the famous E=mc^2.
In space warfare, a cee-fractional strike uses either projectile weapons traveling at near-light speed, or a warship traveling over half the speed of light to attack a target. In the first case, the weapons hardly need warheads, as the tremendous kinetic energy of their high speed translates into a nuclear-scale explosion upon impact. Also, the very high closing velocity makes interception not only difficult, but almost pointless, as the debris will still be incoming on almost the same vector, and even dust or plasma at those speeds can be devastating.
In the latter case, the warship is usually not intended to ram the target (although this was how Erich Von Shrakenberg destroyed Earth's Moon in 2244, and how Scyr assassinated Kazimir Vitek in 2265), but rather to execute a high-speed flyby and strafing of the target, relying on speed to minimize target reaction time, ensure a short total engagment time, and enable an easy escape before a relatively-stationary enemy can accelerate to catch up. Task Force 23 used such a tactic to soften up the orbital defenses above New Madrid prior to invasion in 2243, for example.