The functionality of something is its usefulness, or how well it does the job it's meant to do. You might question the functionality of your new smartphone if you can't get it to send simple text messages.
The noun functionality is often used to talk about technology and the new things it's capable of: "The functionality of the new computer model is incredible!" But you can also use it in a general way, to mean "practicality" or "capability." So you might admire the functionality of your neighbor's very basic, simple backyard grill (as opposed to your fancy gas model), which works perfectly well to cook hamburgers.