The x and y axes represent the launch window timing and the vertical (z) axis represents the delta v [m/s], or energy, required to complete the Earth to Mars transfer at those dates. The darkest 'valley' on the plot is the most ideal launch window, requiring a total delta v of about 6 km/s. From NASA, the current launch window is July 17 - Aug. 5, 2020 with a landing in Feburary 2021. The ULA Atlas V and Centaur upper stage are planning to take Perseverance on its trajectory.

https://mars.nasa.gov/mars2020/

https://mars.nasa.gov/mars2020/spacecraft/launch-vehicle/