Brute Force Attack is one of the most frequently used methods to compromise ones accounts, private data ... basically anything that is protected with a code or password. The way it works, is to automatically generate large amount of possible combinations and test them in order to gain access.
Simple passwords based on one character type only, like repetitive numbers, simple case short words etc ... are the easiest to crack with this method. That's why basically every password protected "place" now requires it's users to pick a password with both upper and lower case letters, including at least one number and a special character.