An outline view hides parts of a buffer to better display the underlying structure.
Related: CategoryHideStuff
Emacs provides several outline features, some come bundled, some must be downloaded.
{{{ and }}}There is also ooutline and noutline which all refer to some version of ‘outline’, so they are not worth mentioning.
If you want basic IDE-style code folding without hours of research:
(add-hook 'prog-mode-hook 'outline-minor-mode)