Cache, not cash (unless you've been on a shopping spree)
It's a local copy of files that the computer keeps to save downloading things again. Sometimes a cache can become corrupt so rather than speeding things up, it slows things down.
For instance, on this site the browser can cache the avatar image and scripts so it only loads the topic content as you browse the site (and that's set to become faster still when it will only load the actual post text, not the headers and other navigation content).