It is a vehicle on an elliptical orbit the crosses both the Earth's and Mars' orbits with transit times of about 5 month in either direction. This allows it periodically to be reached by a shuttle, either from Earth of from Mars. While moving with the cycler, no energy is expended by the shuttle. The cycler can serve as a more hospitable habitat than the shuttles: in particular it can provide better radiation protection from relatively heavy construction, artificial spin gravity and larger amounts of electricity for its hotel load.
The shuttles require about 6km/s of deltaV to reach the cycler from Earth and another 9.7km/s to slow down when they reach Mars, possibly reduced with the aid of skyhooks or the use of aerodynamic capture. On the reverse leg of the journey, the deltaV requirements would be similar.
An alternate approach would involve the cycler with a more efficient but generally mass or energy prohibitive thruster such as a nuclear thermal, Pulsed Fission Fusion(PuFF) or VASIMR thruster. This may reduce the deltaV requirements for the comparably lower efficiency shuttle engines, increasing the effective mass fraction or decreasing trip time.