The maypole and dancing round it forms part of the spring fertility festival to invoke the fertlity of the land. As to what it signifies, I have always been told it represents the penis but this may be just another case of guys trying to ascribe more importance to their genitalia than strictly necessary!. However I have also read that both the Maypole and the Christmas are related to a Norse myth about the an evergreen tree which embraces the universe called teh "Yggdrasill".