The sources check ensures crates only come from sources you trust.
cargo deny check sources
Cargo can retrieve crates from a variety of sources, namely registries, git repositories, or local file paths. This is great in general and very flexible for development. But, especially when re-routing dependencies to git repositories, increasing the amount of sources that a project has to trust may be something a repository wants to explicitly opt-in to.
See Why npm lockfiles can be a security blindspot for injecting malicious modules for the motivating reason for why this check was added.
A project may want to only support local file dependencies, such as having all dependencies vendored into the repository for full control and offline building. This can be achieved by disallowing all git and registry sources to ensure that every dependency is added into your source control rather than via an external source.