Search folders are virtual folders that perform a search on selected criteria once you open the folder, allowing you to collect your items from specific folders or multiple selected folders just by using the right keyword or criteria.

Search Folders have a dedicated section in the left pane below Favorite folders. You can hide this section or show it again from menu > Settings > General > Folders.

Right-click the Search Folders section to create a new search folder. You can also create one using the Search function at the top center - open the drop-down menu and select the 'Create Search Folder' option.

Read more in the Documentation: About Search Folders