Technically, a hit is a request made to a Web server. It is a popular misconception that the term refers to the number of visits a Web page gets, but that is not the case. If a Web page has five images on it, when the page is loaded, six "hits" will be recorded. This is because the browser has to make six requests to the Web server - one for the HTML of the Web page and five for the images.
The term "hit" can also be used to refer to search engine results. When you search for a phrase and the search engine finds 510 results, you can say there were 510 hits and be happy because you used the term correctly.