The width of an object is the distance from side to side. For example, the figure below shows the width of the computer.
In many cases, the width of an object is measured when the "front" of the object is facing you in a way that makes it easy to read or process information.
To measure the width of a book, you will make sure that the front of the book is facing you so that you can read information on the front cover easily.
To measure the width of a car, you will make sure that the front of the car is facing you so that you can see the lights, license plate, ....
To measure the width of a house, you will make sure that the front of the house is facing you.
To measure the width of a keyboard, you will make sure that the front of the keyboard is facing you so that you can read the keys easily. The width of the keyboard below is about 44 centimeters or about 17.5 inches.
Notice that the width of the keyboard is actually the longest side of the keyboard.