Quickly drawing tiles is one of those sections that's really only going to make sense if you've ever done development work on a tile-mapped videogame console, as the mode is quite rare in other applications.

A "tile" is nothing more than an index into a collection of 8x8 graphics squares. If you've played any retro-styled games you've seen the effects of this. They were a great way to deal with memory constraints back in the day, as you reduce 64 pixels worth of on-screen data to a small handful of bytes, describing the index of the tile, and often other attributes like the palette to use, or whether to mirror the tile in a given direction.

Since the Mega Drive can only draw tile-mapped graphics, and has no direct bitmap modes, it makes sense for the rasterization engine in this demo to take full advantage of the hardware. When he talks about drawing an entire 8x8 tile with a one-word write, he means quickly setting the index in this way. Somewhere in memory, he's created 16 solid-color 8x8 tiles, one for each of the palette colors. Then, if any group of 8x8 pixels in a row would be the same color and they're aligned with one of the tiles, he can just point that tile at one of this solid-color indices, and move on. Part of the magic of his algorithm is quickly identifying (in advance) which tiles the megadrive can be quickly drawn with this technique, and which tiles (at polygon edges) need to be drawn more slowly in software.