Words added a prefix a-

"All was a-shake and a-shiver—glints and gleams and sparkles, rustle and swirl, chatter and bubble. "

"All along the backwater, Through the rushes tall, Ducks are a-dabbling, Up tails all!* Ducks’ tails, drakes’ tails, Yellow feet a-quiver, Yellow bills all out of sight Busy in the river!"

Why were these words added a prefix a-?

How to understand?

I have been wondering for a while. Can anyone help? Thank you!