Microinteractions are the little delightful details that are on top of the UX/UI pyramid of needs (if there is such a thing). They are self-contained tiny interactions with one specific task: to provide a small thoughtful touch or a delightful moment. They can employ humor, surprise, emphasis, cheerfulness or more generally a human touch. If a User Interface is the cake Microinteractions are the cherries on top. Microinteractions are what distinguishes a great application/website from an ordinary one.
The time and effort invested in their creation pays off in user engagement and perceived quality of the overall application. Micro-interactions can have many small impacts, from providing instant feedback to your users, enhancing navigation, attracting attention and giving helpful tips. They are the result of a process that had a great human-centered focus as they understand and predict the behaviors of the user clearing up the path from any potential hurdles before and for him. Good application of microinteractions can elevate your application by providing a layer of thoughtfulness and care that the user perceives consciously or subconsciously. They have many applications to individual subcomponents like buttons, call to actions, notifications, and navigation. Like anything good, attention has to be taken as to not overdo it. There is a fine balance between a delightful animation and motion sickness.