A mars cycler is like a space bus that goes back and forth between Earth and Mars. It doesn't stop at each planet, it just keeps moving in an orbit that goes around both planets.
Think of it like a playground swing. You know how you swing back and forth, but the swing's not actually touching the ground when you go back and forth? That's kind of like what a mars cycler does. It goes back and forth between Earth and Mars, but it never lands on either planet.
A mars cycler is kind of like a regular space ship, but it's designed specifically to travel in this orbit between Earth and Mars. It's a bit like a train that travels on a track that's shaped like a big circle.
Why is this useful? Well, if we want to go to Mars, we need a way to get there quickly and safely. A mars cycler is a good option because it could be built to be very fast and efficient. Plus, it would be a lot safer than trying to land on Mars every time we want to visit.