This question already has an answer here:
From "O Captain! My Captain!" by Walt Whitman:
O Captain! my Captain! rise up and hear the bells;
Rise up—for you the flag is flung—for you the bugle trills,
For you bouquets and ribbon’d wreaths—for you the shores a-crowding,
For you they call, the swaying mass, their eager faces turning;
While I cannot recall any concrete examples at the moment, I do remember encountering the prefix "a-" in poetry a couple of times before. Can anyone explain what it is all about?