נָ֥חָה שָׁקְטָ֖ה כָּל־הָאָ֑רֶץ פָּצְח֖וּ רִנָּֽה׃
רויִק באַרוט איז די גאַנצע ערד, זײ ברעכן אױס אין געזאַנג.
Rashi
opened [their mouth] in song This is the song they recited, “All the land rested, became tranquil.”
Ibn Ezra
פצחו They break forth. The root פצח in Arabic means to speak poetically.