A website indexer is a tool that helps search engines find, crawl and store your pages, so those pages can appear in search results. Google keeps a huge web page index, and it only looks there when someone searches. Publishing a page does not add it. Google has to discover the URL, crawl it, and then decide whether to keep it.
That process can stall for weeks, especially on new sites and large catalogs. A website indexer lets you index website pages on a steady daily schedule and shows you which ones are still waiting, so you are no longer guessing.
You may see the same kind of tool called an indexer website or an indexer site, and some people search for a site index website when they mean this too. French-speaking site owners often look for "indexer site web." Whatever you call it, the goal is to index web site content sooner, whether that is one URL or thousands.
Google explains the process in its guide to how Search works, including the fact that it does not guarantee it will crawl or index any page.