The ecliptic is the path the Sun traces against the stars over a year — which is really the plane of Earth's orbit, seen from inside it. Everything in the solar system is built close to that plane, so this one line is where the Sun, the Moon and all the planets are found, always. It is the single most useful line in the sky and almost nobody is shown it.
The ecliptic is not the celestial equator. It is tilted to it by 23.44° — Earth's axial tilt, drawn here as the gap between the two — and that single number is responsible for the seasons, the tropics, the Arctic Circle and the fact that this lesson is worth learning. The tilt is also slowly relaxing, by about 47 arcseconds per century.
The marked point is the top of the ecliptic — declination +23.44°, where the Sun stands at the June solstice. Its altitude here is the highest the ecliptic ever gets in your sky: 90° − your latitude + 23.4°. Six months from now the evening ecliptic crosses the meridian 46.9° lower, which is why winter constellations ride high and summer ones crawl. Precession has dragged this point out of Gemini and into Taurus, where it arrived in 1990 and will stay until the year 4609.
Where the ecliptic crosses the celestial equator you get an equinox, and the Sun is there on about 20 March and 22 September. With the solstice point on your meridian, the March crossing is by definition six hours of sidereal time away — read the azimuth and it is sitting near due west, near 0° altitude. Equator points rise due east and set due west from everywhere on Earth; this is that rule, applied to the most important point on the sky.
The band of constellations the ecliptic passes through is the zodiac — thirteen of them, in fact, since Ophiuchus holds 18 days of it. Follow the line across the sky here: past Aldebaran in Taurus, up over Gemini between Castor and Pollux, straight through M44, the Beehive Cluster in Cancer, and down to Regulus in Leo. M44 sits almost exactly on the line, which is why planets keep drifting through it and why it is worth knowing.
Now the traffic. The Moon's orbit is tilted only 5.15° to the ecliptic, so it is never further than that from this line — and the marked position tells you where it is tonight relative to it. The major planets stay within about 7°. That is the whole trick behind planet-spotting: anything bright and unfamiliar that is not on or near this line is not a planet, and anything that is deserves a second look.
Seven hours. The ecliptic does not stay put: it pivots, because it is fixed to the stars and your horizon is not. Watch the solstice point's altitude fall as the line swings over and the zodiac constellations that were rising take its place. At any instant exactly half the ecliptic is above your horizon — and which half decides which planets you can see tonight.
What this buys you outside. The ecliptic tells you where to look for planets, where the Moon will be tomorrow night, and why eclipses are rare — they need the Moon at one of the two points where its tilted orbit crosses this line, at exactly the moment it is new or full. It also explains the seasons in one sentence: the Sun runs along this line, so in June it is at the top of it and stands high all day, and in December it is at the bottom and barely clears your rooftops.