If someone plays a piece of music cantabile — perhaps on the piano, violin, or flute — it sounds clear and smoothly flowing, like a voice singing. A cantabile passage is usually in a medium tempo and full of feeling.
Cantabile means "singable" in Italian. When you see this instruction in a piece of music, it usually means the melody and style are changing, from fairly fast and complicated to something simpler and with a clear melody — something that sounds easy to sing. The tempo is more manageable by the average singer, and the notes are not too far apart in pitch. Even if not sung, a cantabile passage is played on instruments in a smooth, flowing way that suggests a singing voice.