Hiding a subdirectory means to make it invisible, except for its header line.
dired-hide-subdir). This is a toggle. A numeric argument serves as a repeat count.
dired-hide-all). Or, if any subdirectory is currently hidden, make all subdirectories visible again. You can use this command to get an overview in very deep directory trees or to move quickly to subdirectories far away.
Ordinary Dired commands never consider files inside a hidden subdirectory. For example, the commands to operate on marked files ignore files in hidden directories even if they are marked. Thus you can use hiding to temporarily exclude subdirectories from operations without having to remove the Dired marks on files in those subdirectories.
See Subdirectories in Dired, for how to insert a subdirectory listing, and see Dired Updating, for how to delete it.