Miriam's Cup is a relatively new Passover seder tradition in which a cup of water is set at the table for the prophetess Miriam; in the Bible, Miriam is described as Moses' sister and a leader among Jewish women. This effort can be seen as a feminist gesture toward incorporating female Bible characters into Jewish ritual, which tends to focus more on male prophets such as Moses and Elijah. More strictly religious Jewish people, particularly those who practice Orthodox Judaism, are unlikely to incorporate this new practice into their seder meals, which tend to be more dogmatically traditional.
The significance of using water in Miriam's cup, as opposed to the wine traditionally used in Elijah's cup, is related to stories about Miriam's connection with water while the freed Hebrew slaves were wandering through the desert after their exodus from Egypt. Though the group of freed slaves wandered through the desert for 40 years, they are all said to have survived. This exact tale isn't in the Bible, but it is said that Miriam is the reason the Hebrew wanderers never ran out of water, because God rewarded her personal merits with a steady stream of water that came to be known as "Miriam's Well."