The Earth's tilt on its axis is what causes the change in the seasons and explains why summer days are longer than winter days. The Earth orbits in an ellipse around the Sun, and because of this, it draws closer to the Sun at some points than at others. It is the direction of the Earth's tilt in its axis that determines the length of days and nights.
An ellipse is an oval shape rather than a circle. In the summertime in the northern hemisphere, the Earth is farther from the Sun because of the ellipse in its orbit, but the angle of the Earth's tilt points the hemisphere towards the Sun, making the days longer. The Sun's angle is also higher during the summer months than the winter months. In the winter, the Earth's orbit draws it closer to the Sun, but the Earth's axis tilts away from the sun, making the days shorter in the northern hemisphere. The summer solstice marks the first day of the summer and the longest day of the year. This is because the North Pole is pointed the closest to the Sun than any other day of the winter. The reverse is true during the winter solstice when the North Pole is tilted the farthest from the Sun.