The Ecliptic
The ecliptic is an imaginary Great Circle on the Celestial Sphere along which the Sun appears to move over the course of a year. Of course, it is really the Earth's orbit around the Sun causing the change in the Sun's apparent direction. The ecliptic is inclined from the Celestial Equator by 23.5 degrees. The two points where the ecliptic crosses the celestial equator are known as the Equinoxes.
Since our solar system is relatively flat, the orbits of the planets are also close to the plane of the ecliptic. In addition, the constellations of the zodiac are located along the ecliptic. This makes the ecliptic a very useful line of reference to anyone attempting to locate the planets or the constellations of the zodiac, since they all literally follow the Sun.
The Altitude of the ecliptic above the Horizon changes over the course of the year, because of the 23.5 degree tilt of the Earth's spin axis. This causes the seasons. When the ecliptic (and therefore the Sun) is high above the horizon, the days are longer, and you have Summer. When the ecliptic is low in the sky, you have Winter.
Exercises:
Open the View Options window, and switch to Horizontal coordinates, with the Opaque Ground shown. Open the Set Time window (Ctrl s),and change the Date to sometime in the middle of Summer, and the Time to 12:00 Noon. Back in the Main Window, point toward the Southern Horizon (press S). Note the height of the Sun above the Horizon at Noon in the Summer. Now, change the Date to something in the middle of Winter (but keep the Time at 12:00 Noon). The Sun is now much lower in the Sky.