This is a bowl of sorrel, an herb with a sharp taste. This isn't a common thing for an altar, but we have started to put it on ours at every holiday. Together with the bowl of honey, it reminds us that life contains both bitter and sweet, and we should expect both. It also reminds us that as bitter as it may seem now, there will be sweet later. It comes from a very old tradition, and was shown to me by one of my former students.
After the circle, any leftover sorrel should be taken outside, and left under a tree or plant.