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From :h argument-list:

If you give more than one file name when starting Vim, this list is remembered as the argument list. You can jump to each file in this list.

Do not confuse this with the buffer list, which you can see with the :buffers command. The argument list was already present in Vi, the buffer list is new in Vim.

This subject is introduced in section 07.2 of the user manual.

There is one global argument list, which is used for all windows by default. It is possible to create a new argument list local to a window, see :h :arglocal.

You can use the argument list with the following commands, and with the expression functions argc() and argv(). These all work on the argument list of the current window.