Brute-force is a technique of trying every possible combination when attacking a system. Rather than using a sophisticated method or insider information, you simply start at 1 and try every possible thing. For example if you have a 4 digit password, you can run attacks starting with 0000 and continuing through 9999 until you get in. Secure systems normally take steps to make sure attackers cannot use this method by putting time delays and lockouts into the system.
