A passcode is a secret sequence of numbers, letters, or symbols that you enter to prove you're allowed to access something. Your phone probably has a passcode: you type in those digits to unlock it and keep strangers from reading your messages or looking at your photos. A passcode works like a key, except instead of metal, it's made of information that only you should know.

Passcodes protect all sorts of things: tablets, computers, bank accounts, school lockers with electronic locks, and even buildings with keypad entry systems. Some passcodes are simple, like four numbers. Others are longer and mix letters, numbers, and symbols to make them harder for someone else to guess.

The strength of a passcode matters. Using “1234” or your birthday makes it easy for someone to figure out. A strong passcode combines different types of characters in ways that aren't obvious. Many systems now require passcodes of a certain length or complexity because weak ones put your information at risk.

Unlike a physical key that you can lose, a passcode stays in your memory. But that creates its own challenge: you need to remember it while keeping it secret from others. Writing it down and storing it securely can help when you have many to track.