What is the Torah referring to when it writes "Ka'asher Nisisem ba'Masah"?
Rashi: It refers to when, in connection with the lack of water - Yisrael declared "ha'Yesh Hashem b'Kirbeinu Im Ayin?"
What is the gist of the Mitzvah?
Ramban #1: Not to try out Hakadosh-Baruch. 1
Ramban #2: Not to serve Hashem on condition that He fulfils our requests; otherwise, we will forsake Him. 2
Ramban: Since that is what Yisrael really meant when they tested Hashem in Masa - in that if Hashem would supply them with water in a miraculous manner, they would follow Him in the desert ... . This might have been acceptable before Moshe's status as a Navi had been proven, but once it was, querying him - and subsequent prophets - became akin to querying Hashem Himself. See Ramban DH 'Lo Senasu" who elaborates.