Here’s another plugin I install on every website I create.
Your website will likely have an internal search function — not Google, but a built-in function of the site, with a search field on your sidebar or header. There are always pages that you don’t want to appear in the results. Like the “thank you for subscribing” page, or one you’re tracking statistics on because it’s the landing page linked to from an ad, so you want people to be able to find it only from the ad. And so on.
This seems like it should be a built-in ability of WordPress. Probably someday it will be. In the meantime, I use the Search Exclude plugin by Roman Pronskiy. It adds a checkbox on the editing screen of every page and post, which you check to leave the page out of the search.
This will not also prevent Google and other search engines from indexing the page, however. This is only for the internal search of your site. To make the page private unless you provide a link, you also need to set header information in the page to tell search engines not to index the page. This can be done with another essential plugin, Yoast SEO. Install them both. Go.