Eastern Time is the most populous US zone, running from Maine down to Florida and west as far as Michigan and Indiana. It sits at UTC−05:00 as EST in winter and UTC−04:00 as EDT during daylight saving. The clock above shows whichever applies right now.
ET, EST and EDT are not interchangeable
ET is the year-round name. EST is the winter form and EDT the summer one, so writing "3 p.m. EST" for a July meeting is wrong by an hour in spirit if not always in practice. Using ET, or naming the city, avoids it entirely — see ET vs EST vs EDT.