I believe it's where you batch it and do a block of pixels at once, or a "tile".
This causes speed boosts for lots of different reasons, depending on your implementation and hardware. If you can, check out Ingo Wald's PhD thesis about real time raytracing, he makes a lot of references to batching to get speed using things like AVX (advanced vectoring extensions) in the processor. These provide speed boosts when you calculate the color for a certain amount of similar pixels. It's all cool and definitely worth looking into, and I'm sure I botched the explanation :)
Also when doing real time raytracing, you may be using the graphics card and sending all the data over the bus is a large bottleneck. You don't want to send all the data over to the card to calculate the value of only one pixel because of the bus overhead - so you want to send a reasonable amount of data to chew on while you work on getting the next chunk ready.
I hope this helps.