An embittered situation[1]
ויבאו מרתה ולא יכלו לשתת מים ממרה כי מרים הם על-כן קרא-שמה מרה: ויצעק אל-יקוק ויורהו יקוק עץ וישלח אל-המים וימתקו המים שם שם לו חק ומשפט ושם נסהו
[The Jews] arrived at Marah, and they weren’t able to drink the water at Marah, as it was bitter. Therefore, the place was called Marah. [Moshe] cried out to Hashem, and Hashem showed him a piece of wood. [Moshe] threw the wood into the water, and the water became sweet. There [Hashem] placed for them a decree and an ordinance, and there He tested them[2]
The episode of Marah is short and sweet[3]. The Jews, after traveling for three days, had run out of water. They were hoping to drink from the water sources that they found at Marah. The problem was the water there was too bitter to drink. Moshe threw a piece of wood into the water, and it miraculously became sweet. Right afterwards is a vague verse. The Torah says that Hashem gave the Jews חק ומשפט, a decree and an ordinance. Our Sages teach us[4] this means that at that time they were introduced to a few mitzvos to be involved with: Shabbos, Parah Aduma (the laws of the Red Heifer used for ritual purity), and monetary laws[5] [6]. They would receive the rest of the Torah when they arrived at Mount Sinai.