Icons are a big part of the UI process, they are an inherent part of any design as they visually represent ideas and concepts, sometimes even replacing text in places. If done correctly, you can express the idea behind an action. Most applications, specially mobile ones extensively use icons to communicate with their users.
Types of Icons
It is important to understand the different types of icons we can use in our UI.
FILL AND OUTLINE
We can use fill and outline icons to represent a selected or unselected tab. A really good article that explains a little bit more in-depth about the usage of this type of icons.