This module is an addition to the Search API module which allows users to search on several indexes at once.
When the module is enabled, you'll get a new Views base table where you can search all enabled Search API indexes at once. Performance might suffer, though, if not all indexes lie on the same Solr server (or other backend server that supports this feature).
Please note that, since multi-index searches aren't supported explicitly by the Search API, most other Search API add-on functionality (facets, autocomplete, saved searches, search pages, etc.) won't work with searches created with this module.
Note about Drupal 8
In its Drupal 8 version, the Search API module will allow the indexing of different item types in the same index, thus eliminating the need for this module and providing much better support for this use case. See.