Yes. You can do this on the EDIT PAGE screen for any given web page. You have 2 options:
1. Password Protect your pages (see below)
2. Or, just set your pages to "Private" (you can see this option in the above screenshot as well). This will hide your page, so that it doesn't appear in the navigation menu on your website. This way, the only way someone can find this page is if you (1) link to it from within the text on your website, or (2) you provide them with the link to the page (you can get this link on the WEB PAGES screen, by right-clicking and copying the link location).