Firstly, in order for the transition property to work, the standard property that it will be applied, needs to be defined. Arguably the two most common properties that will be defined are width and height.
Following on, once the property has been defined then the start and end values need to be assigned. In the case of values such as width or height the property needs to be set with a start value and then an end value with some other condition. For example, here David Pickett sets the transition property to width, then the start value of width and then set the end value when the element is hovered over.
Now that he has defined the property to transform, the start and end values, he needs to define the duration of the transition. This is achieved by defining a length in either seconds or milliseconds.