Since the trip to Mars will take a while, the humans need a nice way to stay in shape. Hermes, like many other spacecraft, has a section that spins to make a type of "artificial gravity." Even though I've gone over this several times, let me do a super brief overview of the physics of a spinning spacecraft.

We don't really feel the gravitational force. Instead, we feel other external forces—like the force of the floor pushing on us. This is one of the reasons astronauts feel weightless in orbit.

You can create an external force in the absence of gravity by accelerating a spacecraft.

Since you might not want to keep accelerating and speeding up your spacecraft to simulate gravity, you could instead accelerate by turning in a circle. Moving in a circle at a constant speed is a type of acceleration.

This is exactly the idea of a spinning spacecraft. Spin around so that the humans inside are accelerating and it will mostly feel like it does on Earth.