Index -> Interface -> Filters & Search Options -> Dropdown Filters
Dropdown filters allow you to filter out only cards of a specific rarity, type, class, etc. These filters are off by default. When a filter is on, you'll see a light next to it.
When you access the dropdown menus, left-clicking on an entry will toggle it on or off. (For example: If you access the Rarity dropdown menu, then left-click on R and on U, you will only see Rare and Uncommon cards.)
When you access the dropdown menus, right-clicking on an entry displays cards matching those criteria, while filtering out all others. (Continuing the previous example: If you then access the dropdown menu and right-click on C, you will only see Common cards, while the Uncommon and Rare cards previously displayed will be filtered out.)
