A frame is a screen area that contains one or more Emacs windows; we also use the term “frame” to refer to the Lisp object that Emacs uses to refer to the screen area.
Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).
(selected-frame) ⇒ #<frame firstname.lastname@example.org 0xdac80>
See Frames, for a description of the functions that work on frames.