Highlight is one of the variables you can change with the "set" command. When highlight is non-zero and you receive a personal tell, the sender's handle will be displayed in a different style thus making the tell more obvious on your screen. Other information will be displayed in a different style as well, such as your location in a ranking list or who list.
There are 15 possible styles. However, the way they appear on your screen will depend on many factors. Try they out to see which style works best for you. "Set highlight 0" turns the highlight off.
See also: rank, set, tell, variables, who