Keylogging is an illegal program in which someone can record whatever someone else is typing. The main reason Jagex doesn't allow third-party software, such as autotypers, is that they don't want people complaining that they downloaded a virus or a keylogger when they thought they were downloading safe software.
Keyloggers log your keys to take your password they are mainly used for getting credit card numbers and other personal information. Although some are specifically made to get RuneScape passwords.
If you have a keylogger, do a virus scan, delete it, then change your password and recovery questions.