“Marah” means bitterness

Marah: a place which lay in the wilderness of Shur or Etham, three days journey distant, (Exodus 15:23; Numbers 33:8) from the place at which the Israelites crossed the Red Sea, and where was a spring of bitter water, sweetened subsequently by the casting in of a tree which "the Lord showed" to Moses. Howarah , distant 16 1/2 hours (47 miles) from Ayoun Mousa , the Israelites' first encampment, has been by many identified with it, apparently because it is the bitterest water in the neighborhood.