A prattler is the person who does the talking: they go on and on, in a light, often childish way, about things that do not matter much. The talk is usually not cruel; it is just empty, and it does not know when to stop. You might smile and call a small child a prattler when they chatter about nothing in particular. Said of an adult, it is a little unkind — you are saying they fill the air and never arrive anywhere useful. The word is not common in everyday speech now; it sounds slightly old-fashioned or bookish. People more often say someone is “prattling on” than they name that person a prattler. The verb behind it is prattle; the ending simply marks “the one who does it.”