A password is a word — or a series of letters or numbers — that's kept secret. You might need a password to get into your brother's clubhouse or to use your friend's laptop.
During World War II, soldiers used passwords to enter guarded areas; these were words or phrases (and responses) that only other Allied soldiers knew. Today, you're more likely to use a password for protecting an electronic device, for withdrawing money from a bank machine, or for logging into your email or social media account. The original meaning of password, from the late 1700s, was "word used to distinguish friend from foe."