The word tan sharpens the quality that expresses an adjective. Usually it can be translated as so:
¿Por qué estás tan enfadado? – Why you are so angry?
Often it is used in exclamations, in this case, it has no direct translation:
¡Qué día tan bonito! – What a beautiful day!
¡Qué chicas tan guapas estudian en tu colegio! – What pretty girls study in your school!