delete-selection-mode is an interactive autoloaded compiled Lisp function in `delsel.el'.

(delete-selection-mode &optional ARG)

Toggle Delete Selection mode.
With a prefix argument ARG, enable Delete Selection mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.

When Delete Selection mode is enabled, typed text replaces the selection
if the selection is active. Otherwise, typed text is just inserted at
point regardless of any selection.