This is a bowl of salt, a symbol of the earth. Traditionally, salt has been used for centuries to purify things, and that's what we use it for, as well. It represents the earth on the altar, and is also mixed with the water to bless and purify the circle, the area, and anything that needs to be blessed during a celebration.
After the circle, the salt may be kept, and used again for the next circle. Keep it separate from other salt, though.