The Dired buffer is "read-only," and inserting text in it is not useful, so ordinary printing characters such as d and x are used for special Dired commands. Some Dired commands mark or flag the current file (that is, the file on the current line); other commands operate on the marked files or on the flagged files.
All the usual Emacs cursor motion commands are available in Dired buffers. Some special-purpose cursor motion commands are also provided. The keys C-n and C-p are redefined to put the cursor at the beginning of the file name on the line, rather than at the beginning of the line.
For extra convenience, SPC and n in Dired are equivalent to C-n. p is equivalent to C-p. (Moving by lines is so common in Dired that it deserves to be easy to type.) DEL (move up and unflag) is often useful simply for moving up.