This action queries Obsidian's built-in search and returns the file paths of the notes found. It does not return the notes themselves, just their paths.
Use the "Get Note" action with the returned file paths to get the actual note contents.
The search query is passed to Obsidian as-is, so any search syntax available in Obsidian can be used. See the related official docs for details.
There's currently no good way to programmatically search a vault, and many developers opt to run a search in Obsidian, as you yourself would, and then scrape the results list. This is a brittle process, to say the least, and not working well on iOS, so this action isn't available on iOS.
Most of the time you would use this action to get a list of search results (their file paths), then loop over the results list using the built-in "Repeat With" block. If you want to retrieve the details of the notes, you would use a "Get Note" action and use the loop variable
Repeat Item as the argument for the "File Path or Name" parameter.
If you want to filter the list by folders etc., I recommend using the "Filter List" action from Sindre Sorhus's excellent Actions app (no relation) or a built-in "Repeat With" loop with a contained "If" action.
Changes / History
Removed from iOS. The Obsidian API doesn't support search, so we developers have relied on brittle workarounds to collect search results, but it's not a stable solution, especially on mobile. Please use the new Search Notes With Omnisearch action instead.