'Crackers' are small programs that patch specific software they are written for to disable copy protection or/and to unlock some locked features. These are usually standalone programs that do not do any harm to computer system. Being run they search for specific file (usually with hardcoded name) and change some bytes in it. Most of crackers are packed with home-made encryption/protection routines.
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.