prettify-symbols-mode is an interactive compiled Lisp function in `prog-mode.el'.

(prettify-symbols-mode &optional ARG)

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

When Prettify Symbols mode and font-locking are enabled, symbols are
prettified (displayed as composed characters) according to the rules
in `prettify-symbols-alist' (which see), which are locally defined
by major modes supporting prettifying. To add further customizations
for a given major mode, you can modify `prettify-symbols-alist' thus:

(add-hook 'emacs-lisp-mode-hook
(lambda ()
(push '("<=" . ?≤) prettify-symbols-alist)))

You can enable this mode locally in desired buffers, or use
`global-prettify-symbols-mode' to enable it for all modes that
support it.