What are they?
"Twig can be extended in many ways; you can add extra tags, filters, tests, operators, global variables, and functions. You can even extend the parser itself with node visitors."
{{ post.published_at|date("m/d/Y") }}
There are a lot of places in Drupal that accept Twig extensions. One that I have used the most is inside of views.
If you see "You may use Twig syntax in this field." Or something along those lines, you can create your own!
This page required that the currently viewed microsite's label is being shown.
I am expecting future microsites to have the same sort of page that will need to show it's label as well.