Etiquette, morals, manners, and general good behaviour that all Muslims should strive for.
[14:24] Have you not considered how Allah presents an example, [making] a good word like a good tree, whose root is firmly fixed and its branches [high] in the sky?
[14:25] It produces its fruit all the time, by permission of its Lord. And Allah presents examples for the people that perhaps they will be reminded.
Adab refers to the general rules of Islamic etiquette. In all aspects of his life, especially when dealing with others, a Muslim should maintain himself with respect, dignity, and proper behaviour.
As the prophet said, "The best amongst you are those who have the best manners and character." (Bukhari 4:56:759)