The ecliptic is the path followed by the sun across the celestial sphere during the year. It crosses the celestial equator twice during the year, once at the Autumnal equinox and once at the Vernal equinox. There is no obvious point along the ecliptic at which ecliptic longitude should be zero, and so zero is arbitrarily defined as the location of the sun at Vernal equinox. Ecliptic longitude is then the angular distance of the object eastwards along the ecliptic from this point.
Ecliptic co-ordinates are most useful for solar system objects. For example when an almanac give the longitude of the Sun, ecliptic longitude is meant.