A rogue antivirus program is a fake antivirus program which attempts to convince the computer user to purchase their software.
The program will scare the user by telling them that they have many viruses on their computer when in reality they may not have any. To convince the buyer that they have viruses on their computer, the program may purposely slow down the computer through malicious tools. Many times, it will make the program use up most or all of your sysytem resources to slow down the computer.
It is important to not pay for the program. The program itself will not due anything, plus, it exposes your credit card number which could be used by them for fradulent transactions.