A time zone abbreviation such as EST, CET or JST is a short name for a fixed offset from Coordinated Universal Time (UTC). Many come in pairs: one for standard time and one for daylight saving time, like EST and EDT on the US East Coast or CET and CEST in Central Europe.
The same letters can mean different things in different places – IST here is India Standard Time, but the abbreviation is also used for Irish and Israel Standard Time. Open a time zone to see where it is used, whether it applies right now and the time difference to your location.