In the heliocentric view the Earth goes around the Sun in an orbit which is nearly circular. The orbit can be though of as defining a disk for which the orbit is the rim. The plane which contains this disk is the ecliptic.
If you go outside at noon each day (without using dalight savings time) and plot the location of the Sun on a chart, you will notice that there s a regular progression throughout the year. The Sun is at its highest on midsummer's day, and from then on appears to drop slowly from day to day until midwinter, at which point it is at its lowest point in the sky (if you live in the arctic regions it would be below the horizon in midwinter, that is it will never rise, and you will have 24 hours of darkness). If you draw a smooth curve connecting the points you have plotted, the line will draw out the ecliptic.
The day on which the Sun reaches its highest point is known as the summer solstice (Sun static), the day on which the Sun appears to stop climbing before it starts its slow descent. Similarly the day on which it reaches its lowest point is the winter solstice.
In this picture, a horizontal line drawn midway between the highest and lowest points represents the celestial equator (which lies directly above the Earth's equator). The days when the ecliptic crosses the celestial equator are known as the vernal (spring) and autumnal equinoxs (literally equal night) when day and night are each 12 hours long.