Readable things can be understood, interpreted, or read fairly easily. A readable restaurant menu has print that's large and clear enough for diners to make it out.
Whether or not something's readable is particularly important when you're conveying emergency instructions (like a sign showing how to help someone who's choking) or advertising something (customers can't find your store if they can't read the address in your advertisement). Another way to define readable is "fun and easy to read." Your favorite books, for example, might be fast-paced, readable mysteries.