The upper trigram is made of lines 4, 5 and 6 of a hexagram, the top half. It's the second half to appear when you cast, after the lower trigram.
Why it's called outer
In Chinese it's 外卦, "the outer trigram." Tradition links it with what surrounds you: other people, circumstances, the atmosphere, and also what lies further ahead in time. If the lower trigram is your starting point, the upper one is the ground you're stepping out onto.
How to use it
Read the two trigrams as a scene. In Decline, Heaven is above and Earth is below. It looks like the natural order, but heaven rises and earth sinks, so they drift apart and never meet. In Well-Being they're the other way around, and that's why they come together. The same pair of trigram figures, swapped, tells the opposite story.
Within the upper trigram, line 5 is the center and, by tradition, the ruler's place: it's often the strongest line in the hexagram. Line 4 sits close to that power, like an adviser. Line 6, the highest, often speaks of something that has gone past its peak or stands off to the side. All of this is covered in line positions.
A practical tip: when the upper trigram looks hard, ask whether that part depends on you or on your surroundings. The answer often tells you where to put your energy.