Computers do numbers. Just numbers. They don’t do pictures, sounds or text. To represent all these other things computers use a series of numbers, and to ensure that everyone knows what everyone else is talking about the same series of numbers is used for the same thing. A WAV file, for example, uses a series of numbers to store sound. The format of the file is well defined which means that any computer can read a WAV file created on any other computer.
Text works the same way. Characters are encoded using a number. The actual number doesn’t really matter as long as we know what the number is and everyone uses the same one.