This represents time zones. It means (usually) 9 pm in the Eastern Time Zone (and usually Pacific Time) or 8 pm Central Time. The c stands for Central.
TV networks like to use the same advertising for all regions so they use this shorthand terminology. Historically, the Eastern and Central zones were on the same feed, so the pre-recorded shows occurred simultaneously in both regions, meaning that in local time, everything was an hour early in Central. Similarly, Pacific and Mountain times were paired.
With improvements in technology, these pairings aren't necessary so Mountain and Pacific often follow the same times as Eastern... But not always. It can depend on the show or network, which is why the most reliable interpretation only relates to Eastern and Central.
Note that live shows, particularly sports, will always be shown simultaneously everywhere so if a game starts at 9 pm Eastern it will air in the other regions at 8, 7 & 6 pm as you move west.