The first stanza means "your world is as big as you make it." It talks about how a girl is very abide. In the stanza she says that she hides in the narrowest nest in a corner, this means that she is very timid.
The second stanza means " To travel where you can go." It talked about how a girl had a burning desire to travel while as she looked out at the horizon by the sea.
In the 3rd stanza is talks about how she's finally free and is now open in the world and no hiding in a corner , she felt happy and powerful.