Rust crate
A Rust crate is the unit a piece of Rust software is delivered and named in: one library, under one name, at one version.
In more detail
A project keeps a short file listing the crates it wants and the version of each. The build tool reads that list, fetches them, works out what they themselves want, and compiles the lot together into a single program.
Because everything is compiled together, what comes out is one file with nothing to install beside it. The price is that the build takes the time it takes; the gain is a program that carries its own dependencies inside.
What it does on a real document
The engine is a crate, and the Rust side of it is used that way, with one difference: it is on no public registry. A project names the copy you were given by its path on disk, or fetches it from a private registry of your own, and the engine's code ends up inside your own program.
Everything the engine needs to draw travels with it, so a program built on it reaches nothing over the network while it works.
Where it comes from, and why
The word was chosen for the image: a crate is what a delivery arrives in. It has named the unit since the language's early years, and the shared store it is fetched from opened in 2014.
A crate has a version written as three numbers, and a project says which of them it will accept. That is what lets a library be improved without breaking the programs already built on it.