The term "optimistic UI update" refers to the approach of immediately updating UI to reflect user actions rather than waiting for a success response from a server. This also requires reverting state in the event of a failure. A real-word example of optimistic UI updates in action is liking and unliking a tweet on Twitter. You can see this yourself by going to Twitter on their desktop web client, opening your devtools, enabling "Offline" in the "Network" panel, and liking a tweet.
In this video, we'll compare the look and feel of non-optimistic UI updates with their optimistic equivalent and introduce the liking and unliking a tweet example we'll be building.