This plugin is part of a suite of plugins that comprise the Backstage search platform, which is still very much under development. This plugin specifically is responsible for:

Allowing other backend plugins to register the fact that they have documents that they'd like to be indexed by a search engine (known as collators)
Allowing other backend plugins to register the fact that they have metadata that they'd like to augment existing documents in the search index with (known as decorators)
A scheduler that, at configurable intervals, compiles documents to be indexed and passes them to a search engine for indexing
Types for all of the above

Documentation on how to develop and improve the search platform is currently centralized in the search plugin README.md.

For a better overview of how the search platform is put together, check the Backstage Search Architecture documentation.