Airmail allows you to hide/unhide the folders/labels. This feature allows you to hide/unhide them only in Airmail and this action will not make any effect on the web-client.
Right click any folder/label in the list and a dropdown menu will appear, choose hide folder to hide it.
You will notice that the hidden folders are now in a faded color. Please see the picture below.
To completely hide them from the list, choose 'Hide Hidden Folders' from 'Folders' on the menu bar as shown in the picture below:
If you want to hide some of the folders which are above the labels/folders list such as starred, important, snooze, please see 'Favorites'.