EDT stands for Eastern Daylight Time. CST is known as Central Standard Time(now Central Daylight Time CDT). CST is 1 hour behind EDT.
So, when it is EDT at 10 a.m., it will be CST at 9 a.m.
Download a world clock app, so you don't have to convert between time zones over and over again.
In the central time zone it is one hour earlier than in the eastern time zone. Normally both zones are either on standard time or daylight savings time.