In brief
transition describes how a change between values unfolds over time. An opacity transition becomes visible when opacity actually changes between states.
Useful to know
transition describes how a change between values unfolds over time. An opacity transition becomes visible when opacity actually changes between states. [1]
The rule does not create starting and ending states for you. If motion is absent, check the property, differing values and duration. Limit transitions to the needed properties to avoid animating unrelated changes.
Sources
For further detail and to check the information: