org-agenda-files

org-agenda-files is a variable defined in `org.el'.
Its value is

nil

Documentation:
The files to be used for agenda display.
Entries may be added to this list with M-x org-agenda-file-to-front and removed with
M-x org-remove-file. You can also use customize to edit the list.

If an entry is a directory, all files in that directory that are matched by
`org-agenda-file-regexp' will be part of the file list.

If the value of the variable is not a list but a single file name, then
the list of agenda files is actually stored and maintained in that file, one
agenda file per line. In this file paths can be given relative to
`org-directory'. Tilde expansion and environment variable substitution
are also made.

You can customize this variable.