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 section Visiting Files.
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 section Multiple Windows.
dired-mouse-find-file-other-window). This uses another window to display the file, like the o command.
dired-view-file). Viewing a file is like visiting it, but is slanted toward moving around in the file conveniently and does not allow changing the file. See section Miscellaneous File Operations.