The meme stems from a lovable costumed feline whose owner gets it to play a keyboard. By cutting it into any video, it provides a humorous effect, a cross between the "wrap up your acceptance speech" music at the Academy Awards, and the large hooks they used to use to drag off actors in vaudeville shows.
If you've been itching to create your own Keyboard Cat video, but don't have the software to do it, fear not. There's Webware for that. Bobsworth Industries, which makes some other fun Web tools, has put together the Auto Keyboard Cat.
You just drop in any YouTube URL, and pick when the cat should come on. It then streams the two clips together. You can then pass that URL on to a friend who sees both clips together.
It's missing a few things I'd love to see, such as the option to take that video and embed it elsewhere. You also can't play any more of the source video once the Keyboard Cat starts playing. Some of the most masterful works on YouTube use this technique and it's quite funny--although again, this tool simply combines the two clips together.