The document discusses English prepositions of time - AT, ON, and IN. AT is used to indicate very specific times like hours (e.g. "at 6 PM"). ON is used to indicate days (e.g. "on Friday"). IN is used to indicate less specific longer time periods like weeks, months, years, and decades (e.g. "in July", "in the 80's"). Examples are provided for how to use each preposition of time correctly in sentences.