What is the meaning of rock a bye baby?

(author unknown)

Rock-a-bye, baby

In the treetop

When the wind blows

The cradle will rock

If the bough breaks

The cradle will fall

And down will come baby

Cradle and all

Baby is drowsing

Cozy and fair

Mother sits near

In her rocking chair

Forward and back

The cradle she swings

And though baby sleeps

He hears what she sings

From the high rooftops

Down to the sea

No one's as dear

As baby to me

Wee little fingers

Eyes wide and bright

Now sound asleep

Until morning light

It's referring to a Native American woman that a pilgrim saw when she was hanging her baby in a cradle onto a tree branch so that the wind would rock it to sleep.