EST means Eastern Standard Time, UTC−5. EDT means Eastern Daylight Time, UTC−4. New York, Toronto and Miami use EST in the winter and EDT from the second Sunday in March until the first Sunday in November.
Because "EST" is used loosely all year, an invitation for "2pm EST" in August is genuinely ambiguous: some people mean 2pm as the clock reads in New York, others mean 3pm.
The safe habit
Write "Eastern Time" or "New York time" and let the reader's calendar do the conversion. If precision matters, add the UTC equivalent in brackets.