Laid means "set down." If you built a brick wall, and then when it's done your neighbor complains that the wall crosses onto his property, tell him, "too late! The brick has already been laid."
Laid is the past participle of the verb, lay, which means set down. So something that has been laid has already been set down. You might scramble up the eggs the chickens laid yesterday. Before your guests come over, your table should have been laid. Or you might examine the foundations that the builder laid down for the house you're building. We often use laid if we want to emphasize how carefully something has been done.
