`restore-keys` is a configuration option for the actions/cache action which specifies fallback behavior. The [docs][docs] say it best: > When a cache miss occurs, the action searches for alternate keys > called `restore-keys`. > > If you provide `restore-keys`, the `cache` action sequentially > searches for any caches that match the list of `restore-keys`. > ... If there are no exact matches, the action searches for partial > matches of the restore keys. When the action finds a partial match, > the most recent cache is restored to the `path` directory. So this improves caching when there's a miss. For example if I edit `.github/workflows/languages.toml`, the current behavior is that the cache for downloaded grammars will miss and all of them will need to be fetched again. With `restore-keys`, we use the latest published cache as 'good enough', we'll fetch whatever grammars changed, and then at the end we publish a new cache under the new hash. [docs]: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#example-using-the-cache-action