When you come across the instruction dolce in a piece of music, play or sing it in a sweet and gentle way, which means it should also be fairly soft or quiet.
Dolce is Italian. It’s hard to pack its meaning into just one English word, but "sweet," "gentle," "soft," and "tender" all come to mind. In music it describes an overall feeling or style, usually including the fact that the music is a bit quieter and not too fast — in other words, much like you’d expect a lullaby to sound. Dolce can be used as an adverb, as in "This piece of music should be played dolce," or an adjective, as in "Next comes a dolce section."