The lower trigram is made of lines 1, 2 and 3 of a hexagram, the bottom half. Since lines are counted and cast from the bottom up, it's also the first half to appear when you consult.
Why it's called inner
In Chinese it's 內卦, "the inner trigram." Tradition links it with what's close at hand: your inner world, your attitude, what you have to work with, the ground everything rests on. Its partner, the upper trigram, is the outer one: the surroundings, other people, what's coming.
In very old texts, such as some consultations told in the Zuozhuan, the lower and upper trigrams had names of their own. Today it's enough to think of "inside" and "outside," or "beginning" and "unfolding."
How to use it
Look at which trigram sits below and ask what it says about you or your starting point. In Inaction, Heaven is below: there's strength and a wish to move ahead. Water, danger, is above. The strength is real, but the moment asks you to wait.
Within the lower trigram, line 2 holds the center, a place the tradition values highly. Line 3 sits at the edge, about to cross into the upper half, so its texts often speak of strain or transition. More on that in line positions.