The Browser pops up anytime there's a lot of options to choose from. Be it when inserting new Blocks, Fixtures, loading Block Stacks and other situations.
You can select an option by clicking on the squares. When the square displays "...", it's a category folder and will open that category for further selection.
You can search by typing on the keyboard. Pressing the Escape key leaves the last selected category or eventually closes the browser and cancels the action, just like pressing anywhere outside the displayed options.
If you find yourself browsing for some options over and over, the Favorites feature might come in handy.
Using Ctrl+Left click you can set an end option (does not currently work for categories) as favorite. You will immediately see the option receive a golden border.
Pressing Tab switches between standard browsing and viewing favorites.