There are several Dired commands for visiting or examining the files listed in the Dired buffer. All of them apply to the current line's file; if that file is really a directory, these commands invoke Dired on that subdirectory (making a separate Dired buffer).
dired-find-file). See Visiting.
dired-find-file-other-window). The Dired buffer remains visible in the first window. This is like using C-x 4 C-f to visit the file. See Windows.
dired-mouse-find-file-other-window). This uses another window to display the file, like the o command.
dired-view-file). View mode provides convenient commands to navigate the buffer but forbids changing it; See View Mode.
dired-up-directory). This is equivalent to moving to the line for .. and typing f there.