Laura stretch'd her gleaming neck Like a rush-imbedded swan, Like a lily from the beck, Like a moonlit poplar branch, Like a vessel at the launch When its last restraint is gone.
Laura is craning her neck to see the goblin men better, and the poet compares her to a swan leaning out of the rushes in a stream.
Then she is compared to a lily that leans over a "beck" or a brook.
Then she's compared to a delicate kind of tree branch on a "moonlit" night.
Finally, she's compared to a ship that's just leaving dock. The ship starts to move forward when the anchor is pulled up and all the lines are in.
The stanza ends with the words, "when its last restraint is gone." This phrase refers to the ship that Laura is being compared to. It literally means that the anchor is up and the ship is untied and ready to go. But this line could also suggest that Laura's guard is down – she's unrestrained. Anything could happen.