It is the title by which Archdruid Malfurion Stormrage is addressed, both by druids and by others, such as Warden Maiev Shadowsong. It was also the title by which Malfurion addressed the demi-god Cenarius, and occasionally used by Illidan whilst under Rhonin's tutelage during the War of the Ancients.
The complementary honorific is thero'shan.
As the word shan is present in both words, it likely means honored in Darnassian. Given that Shan'do means "honored teacher" and Thero'shan "honored student", the placement of Shan first in Shan'do may indicate an emphasis on honored reflecting the teacher's superiority to the student.