Some passages of music are tense and exciting, while others are tranquillo — calm, quiet, and relaxed. Tranquillo is the word used to mark those calmer passages in the sheet music for performers.
Tranquillo is Italian, but it looks similar to the English word tranquil, meaning "peaceful, quiet, and serene." That's because both come from the Latin word tranquillus, which has the same meaning. Tranquillo can be used as an adverb, as in "Play the next several bars tranquillo," an adjective, as in "There’s a tranquillo passage in the middle of the piece," or a noun, as in "Let's take it from the beginning of the tranquillo."