At the risk of telling you the blindingly obvious, project search allows you to search for documents inside your project that contain a particular word, phrase, label, keyword and suchlike. Just click in the search field on the right-hand side of the toolbar and type the phrase you want to search for. (When clicked on, the downwards-pointing triangle next to the magnifying glass in the search field brings up a menu that allows you to modify the search options.) Click in the search field now and type “collection”.
Note how the binder is replaced by a darker coloured “Search Results” list. This is a list of documents that contain the word “collection” somewhere within them. Also note that the search term—in this case “collection”—is now highlighted in yellow in the current document wherever it appears. So you can click on the documents in the search results list to and easily see at a glance where the term you searched for is in the document. For longer documents, just click into them and go to Edit > Find > Find… (or hit Ctrl-F) to bring up the Find panel, which can be used to search within the documents themselves. To get rid of the search results list, you can click on the “close” button on the right of the search field:

Or on the button in the footer bar of the sidebar:

Do so now, and then move on to “Step 13: Collections”.