User authentication, the process by which the identity of a person is verified, often relies on what the user knows (a password), what the user has (a card or physical key) or what the user is (a fingerprint). Password authentication is prevalent and widely used by modern applications and systems, making strong passwords important as they provide a higher level of security and decrease susceptibility to identity theft.
A strong password typically meets conditions of adequate length and has a sufficiently large number of different characters. These conditions are not arbitrary, neither are they some impenetrable consequence of divine law.
Just imagine a safe or a combination lock. If your high school experience was not atypical, your locker's lock probably had two digit numbers around the circumference of the combination dial. A lock with 3-digits would have a 10-fold increase in possible combinations or passwords. The more possible combinations, the harder it is to guess the correct one
Computers don't make much of a distinction between numerals, punctuation and letters, but people do. When attempting to guess your password, a sly attacker may attempt only the memorable or most common combinations (so don't use your birthday or dog's name.)
Humans are better at memorizing words than we are at memorizing strings of numbers or special characters. The best way to beat any hackers is to use a strong password which contains a mix of letters, numbers and special characters which would incur greater difficulty in memorization.
Of course if you don't want to remember passwords, there are software and even paper-based solutions to password management. Programs like 1Password, KeePass, Mac OS's Keychain, or GNOME Keyring can make it much easier to use secure passwords across you journey on the internet, just be sure they're locked down by a strong password.