People need a way of interacting with machines if they are to be useful.
Think of a vending machine - you want a drink, but how do you get the machine to give it to you?
Well, you will have to read the display and follow the instructions, put your money in the slot, press the right buttons and eventually your drink will pop out (hopefully!).
You got the machine to give you a drink by interacting with it via its 'user interface'.
Think about all of the other machines that you interact with on a daily basis. You have to do certain things and make choices to get them to work.
We will be looking at different types of user interface over the next few pages.