Elysian Shadows

SDL_FillRect(buffer, &buffer->clip_rect, 0x666666);

I think it means make a rectagle (the size of the buffer, the positon of the buffer, the color); am I close?

You should really learn to use google. So far, almost everything that you've asked about would be easily answerable using google. To answer your question, it fills in an SDL_Rect with the color that you tell it to. The first argument tells it where to fill in the rectangle. The second argument tells it how big of a rectangle to fill in. The third argument specifies the color to fill the rectangle with. In this case, it is filling the whole buffer with a grayish color.

http://en.wikipedia.org/wiki/RTFM

the function you asked about.
http://www.libsdl.org/cgi/docwiki.cgi/SDL_FillRect

downloadable documentation.
http://www.libsdl.org/archives/sdldoc-html.zip

Oh god... PLEASE don't post if you're just going to ask a simple question like this.
Learn to use Google, it is your best friend.