The reaction of an alkyllithium compound with an arene bearing a "Directed Metalation Group" (DMG) normally leads to an ortho-metalated intermediate. Good DMG's are strong complexing or chelating groups that have the effect of increasing the kinetic acidity of protons in the ortho-position.
The ortho-metalated intermediate can be reacted with a variety of electrophiles, after which the DMG can be retained if desired, converted to a different functional group, or in some cases removed.