I've been trying to find an answer but any search for views embed display brings up a bunch of unrelated modules and questions.
There is a Views Display Type/Mode/Whatever called "Embed" that you select when creating different views in your view:
For what can I use this display type?
This is a display which can be embedded via views_embed_view() or drupal_view() of Twig Tweak.
You can embed other display types, too, for example Page or Block, but the first one generates a route and the second one a block derivate you see when adding blocks to the block layout. So Embed is basically a display type which does nothing until you embed it in code.
The only explanation you can find in Views UI is in the general settings, where there is this checkbox:
[x] Allow embedded displays
Embedded displays can be used in code via views_embed_view().
Which by the way is without function and there is an open issue to remove it.
