The orbit of Mars is not quite circular, it is slightly elliptical (an ellipse is a "flattened" circle). The sun is not in the center of this ellipse, but is offset to one side. As a result, there is one point in Mars' orbit where it is closest to the sun (called perihelion) and one point where it is farthest (called aphelion).

The Earth has seasons because the tilt of its rotational axis is inclined with respect to the plane of its orbit. During the northern summer, the northern hemisphere is pointed towards the sun. During the northern winter, the southern hemisphere is directed towards the sun. Mars has seasons because its axis is tilted as well, 29.15° to be exact. Mars is at perihelion during the southern summer, making it slightly more intense than the northern summer.