An engine block in a box. Something like that. Crate engines are replacement engines. For our vehicles, they are often overhauled or made up of new parts. And that build-up does not even have to have been done in accordance with the original factory specifications. The factory may not have had anything to do with it.
Due to the improved production and / or finishing possibilities, a crate engine can be even better - or considerably faster - than the original. An approach that, for example in the Mopar and Chevrolet corners, is already top-notch.
But the best thing about a crate engine is of course unpacking and installing. And then starting it for the first time.