The standard passcodes that lock the iPad are either 4 digit or 6 digit numbers. You can set a harder to crack, custom passcode/ passwordin the settings using letter and numbers if you so desire.
A password is a series of numbers, symbols and letters that a user sets up in order protect information in a setting, a web site, email, etc. This can actually be a word or series of words, if you wanted to set it up that way.
Your iPad is looking for that 4 or 6 digit code. If you can not remember it, you have to restore the device in order to unlock it.
If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support