The basic initial / default query looks like this:

It would do the following

1. The Cache is usually often the root of every query, as it provides all the items in an App that have been cached. 
2. The Wirings connect two DataSources. In this case the Out of the Cache to the In of the Publishing filter. For performance reasons, the Cache already caches which items are published and which are draft, so that the publishing filter will be faster. 
3. The Publishing filter detects if the user is a visitor or editor, and will provide the right items to anything that follows it. Editors can see drafts, while visitors won't see them.
4. The Module DataSource picks the items that were manually added to a module. This is actually the default setup for views which have a Content-Management style, where a user manually adds pictures and texts. For data-processing queries, you'll usually remove this. 
5. The data is then forwarded to the Target which is usually a server-side template or a REST call (for use in JavaScript)
6. To the right you can see Params of this query. By default it shows you some dummy params so that you can learn about them. 
7. You can also see Test Parameters. These are used when you hit Run on top, so that you can test the query and simulate various inputs.