Here's an example: You make some toast. You don't want to just plop it on the counter because you haven't cleaned that in ages because of poisonous cleaning agents and the cat probably walked on it, so you put it on a plate.
When you're done with your toast, all it has is some crumbs and a schmotz of organic margarine. The plate is still kinda clean. So you wipe it off on your pants and put it in the Clerty pile. You can use it again later. Keep using it until the taste of whatever you had on there before is affecting what you're eating now. (I do hope that was the proper use of the word, affect.)
At which point, it's time put put that plate in the dish washer.
My next green tip will explain why you should be using the dishwasher. Stay tuned!