du gives the size of a file or a directory (=the size taken by it on the disk).

By default, the command is recursive: it executes on the current directory and all its sub-directories.

du options meaning
-h print sizes in human readable format (e.g., 1K 234M 2G)
-s display only a total for each argument
-k display in kilo-bytes
-x skip directories on different file systems