Archaeologists suggest that the statues were a representation of the Polynesian people’s ancestors. They face away from the sea and towards the villages, like watching over the people. There are a lot of theories how did those statues appeared on their places. In 2013, scientists claimed they may have been walked into position. They created a five-tonne replica of one of the statues and moved it into an upright position on a dirt path in Hawaii, to prove that the statues could have been 'walked' into place.
The article on Mystery of Easter Island will help you to make a deeper research of the subject, even though it is not actually ‘mystery’ as for me.