This is a little bowl of water. In this case, the bowl is varnished wood, but it can be anything, of course. All four elements are present on the altar at all times, and water is one of them, so here it is. This is also the bowl that is used to mix the salt and water in to bless the circumference of the Circle, and for any blessings that are part of a celebration. In our own tradition, we have collected water from all over the world, and a few drops of that water go into this bowl at every circle (and are replaced by water from the bowl) to remind us that all of the water all over the planet is really one body of water, and everything that is done to any affects it all; just as we are all one people and anything that is done to any one of us affects us all. We call this "Water of the World." We also use it to bless the Earth, since it comes from all over.
After the circle, it's traditional to empty this bowl into running water. It's best if it can be emptied into a stream outside; but in these days many of us live far from streams or even drainage ditches with running water, so it winds up being emptied into a stream of water running from the tap. Eventually, of course, that stream will join with running water outside, so we use it if we must. After all, we tend to be a practical people!