The EIT (Event Information Table) is extra data that is broadcast with a television signal, much like Closed Caption Text in the VBI stream. EIT provides program data for the current show and future shows which can be then used to view on-screen program information, such as title, length, description and more. This can also be gathered to build a program guide for current and future shows on all your available channels. MythTV uses an Electronic Program Guide, or EPG. Most cable and satellite services do. Even many modern digital televisions can have a program guide by gathering and storing EIT.
EIT is typically available over digital television signals, either terrestrial, cable or satellite. With terrestrial signals, each channel provides it's own EIT. Information is usually only available for the upcoming 24 hours. With satellite, the EIT is broadcast on it's own stream, or an Aggregate Event Information Table (AEIT), and contains information for all the channels broadcasted, and can provide scheduling information for more than a week.
(Note for DVB-T users (I am in the UK): EIT only seems to work when channels have been scanned by MythTV. Importing channels.conf doesn't provide enough information for MythTV to use EIT).