The words “come hither” mean “come here,” because the singer is describing how peaceful it is in the country under the greenwood tree compared to the Duke’s Palace where they lived before the duke’s brother sent them all into exile.
The poem “Under the Greenwood Tree” is actually a song from the play As You Like It. In the play, a duke is sent into exile by his evil brother. The followers of the duke gather in Arden Forest, where they find the scene very peaceful. The song is sung by a character named Ainiens, who certainly seems to enjoy the slower, more peaceful existence of the forest.
Come hither, come hither, come hither;
Here shall he see
But winter and rough weather. (Act 2, Scene 5)
The song is short, but it means that they have no enemies but the weather, as opposed to the duke’s enemies and their enemies in the palace. The song is a celebration of the fact that they are free, away from the palace intrigue, and relatively safe for now, away from "ambition."