To edit a file, you must tell Emacs to examine the file and prepare a buffer containing a copy of the file's text. Emacs makes a new buffer for each file that you visit. Most Emacs commands that operate on a file require you to specify the file name. File names are specified using the minibuffer.
Completion is available, to make it easier to specify long file names.
Each buffer has a default directory, normally the same as the directory of the file visited in that buffer. When Emacs reads a file name, if you do not specify a directory, the default directory is used. The default directory actually appears in the minibuffer when the minibuffer becomes active to read a file name.