EST is considered a shorthand for the following applicable words: Estimate, Established, or most commonly Eastern Standard Time. Depending on the context clues you find your form of EST used in, one can approximate the word being used.
In this instance, “Stream M-F 10 PM EST” is a very concise way of naming a time for a livestream. M-F is shorthand for Monday through Friday. PM stands for Post Meridian, After Midday, which is used for telling time that differs from the Military or 24 hour standard time keeping. Considering the majority of the designation refers to schedules and time, it would be reasonable to assume that EST in this context refers to Eastern Standard Time which is the time zone for places like New York City and is considered 5 hours behind Coordinated Universal Time.
But considering I’m sometimes a bit frackin lazy, EST might as well mean Estimated because I walk into the stream anywhere from several hours ahead to an hour late or just whenever I freakin feel like it.