This means that Internet service providers and others between you and the website cannot insert ads in web pages. It also means that if you’re reading a newspaper online (for example), you can be sure that the articles you read really are from that newspaper, that all the articles are there and that they were not modified.
It also prevents URL or content-based censorship, since mans-in-the-middle cannot know what URLs you visit or what content you received from a website.
It does not prevent domain name or IP-based censorship, since the man-in-the-middle can know these, and it is also sometimes possible for a man-in-the-middle to know what page you are visiting by examining the length of the URL and content and comparing them with public information available on the website.