The transition  property is used as see in the previous video:

transition: WHAT DURATION DELAY TIMING-FUNCTION; 

Example:

transition: opacity 200ms 1s ease-out; 

Can be translated to: "Animate any changes in the opacity  property (for the element to which the transition  property is applied) over a duration of 200ms. Start fast and end slow, also make sure to wait 1s before you start".

Instead of this shorthand, you can also specify the four individual properties:

1) transition-property  (https://developer.mozilla.org/en-US/docs/Web/CSS/transition-property => transition-property: opacity; 

2) transition-duration  (https://developer.mozilla.org/en-US/docs/Web/CSS/transition-duration) => transition-duration: 200ms; 

3) transition-timing-function  (https://developer.mozilla.org/en-US/docs/Web/CSS/transition-timing-function) => transition-timing-function: ease-out; 

Possible timing function values are: ease-out , ease-in , linear , cubic-bezier()  and a couple of others. See the above link as well as the next lecture for more details.

4) transition-delay  (https://developer.mozilla.org/en-US/docs/Web/CSS/transition-delay) => transition-delay: 1s; 

You can read the official MDN article on CSS transitions here: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions