The number of hits on a server may be important for determining advertising revenue.
In the course of loading a single web page, a browser may hit a web server many times e.g. to retrieve the page itself and each image on the page. In contrast, caching by browsers and web proxies reduces the number of hits on the server because some requests are satisfied from the cache.
3. <jargon> To press and release a key on the keyboard. Some prefer the less aggressive "tap".