The word
大人, dàrén, means "great person." In ancient times it was someone with authority: a ruler, a high lord. In the I Ching it's also someone of wide vision and integrity, who not only acts well but can guide others. If the noble person is the model of conduct, the great person is that model with the reach of a guide.
The formula
It appears mostly in one fixed phrase: 利見大人, "it is favorable to see the great person." You'll find it in the second and fifth lines of Activity, and in the judgment of Conflict (6), Held Back (39), Meeting (45) and Gentleness (57). In Going Up it says something close: "Seeing the great person is of use."
The Wenyan describes the great person of the fifth line of 1 as someone in harmony with heaven and earth.
How they change
In Renewal: "The great person changes like the tiger." When the one who guides changes, the change is visible and complete, like a tiger's stripes after it sheds its coat.
In a reading
When this formula comes up, the practical question is: whom should you consult? It might be someone more experienced, someone with authority over the matter, or a good adviser. Sometimes the reading points the other way: in this matter, it's your turn to be that person for others.