Pretty much the obvious - they are drop in engine swaps sold in a crate. These offer a somewhat straightforward way to upgrade a powertrain and often come with brand recognition (like the "Hellcrate"), certifications to a certain power level, pre-made engine control tuning, or sometimes even a warranty.

However, in the internal combustion world, they generally represent a poor value compared to sourcing a used engine block and upgrading it with aftermarket parts. So they're by and large the domain of wealthy buyers who are usually paying a shop for a "build."