Commerce Search integrates Drupal Commerce and Drupal core's search.module to expose products as a search option alongside "Node" and "User" in the search settings form and search results.
This is useful to:
integrate full text search of products into an existing node-based or Comerce Product-based Views module view, without needing to rebuild the view as a Search API Index-based view.
enable product-only search without depending on Search API nor a Solr server.
Modules that use Search API and/or Solr are more powerful and flexible:
Commerce Search module, however, integrates with Drupal core's search.module, so it is easier to get running quickly, and integrate with node-based Views module views that would otherwise be difficult to create as or convert to Search API index-based Views module views.