A Mars cycler is a hypothetical transportation system between Earth and Mars.
The cycler is a vehicle on an elliptical orbit the crosses both the Earth's and Mars' orbit. 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 an habitat, and in particular provide radiation protection from relatively heavy construction, compared to the shuttles.
The shuttles require about 6000 m/s to reach the cycler from Earth and another 9700 m/s to slow down when they reach Mars. However, the deltaV at Mars could be replaced by aerodynamic capture, saving on propellant. On the reverse leg of the journey, the deltav requirements would be similar.
The transit times between the two planets would be about 5 month in either direction.
The Mars cycler was proposed by Buzz Aldrin as a way to reach Mars more economically.
- Wikipedi Mars Cycler https://en.wikipedia.org/wiki/Mars_cycler