A crush washer is a washer made of soft material (like aluminum) that gets squished when tightened so that it conforms to any irregularities in the finish of the oil pan or the drain plug itself. It's a one-use washer, since once it's squished, it can't be squished again.
With the Fumoto, I just used the washer that came with it. Since the valve is installed once, there are no washers to replace with each oil change. But the best thing is that I have one of the "N" model valves with the nipple for connecting a rubber tube. This allows you do dress the tube in such a way as to drain into your drain pan without splashing and making a mess.
As you can imagine, 0W-20 oil is extremely thin when hot, and so it comes out FAST when removing the original drain plug. The Fumoto/tube combo gives you the control you need to avoid this mess. And if you ever want to submit an oil sample for analysis, the Fumoto makes it easy to take out a controlled amount of oil for testing.