Jess makes the wreath to honor Leslie.  He takes it to the sacred grove, and leaves it there in her memory.

Jess's actions in relation to the wreath are significant for a number of reasons.  First of all, it shows that he has accepted and come to terms with Leslie's death.  Also, in taking the wreath to the sacred grove alone, Jess shows how he has grown.  Jess had always been somewhat afraid of venturing into the darkness of the grove, and in going there by himself he is facing his fears and conquering them.  He says the magic words to make Terabithia come alive without Leslie to support him, and in returning, he again conquers his fears to help his little sister, who is stuck in the middle of the creek, get back to the other side.  Jess had always thought that fear was a sign of weakness, but he now knows that it is a natural feeling that is common to all people.  Through the experiences he shared with Leslie at Terabithia, Jess has learned "to see beyond the shining world - huge and terrible and beautiful and very fragile", and found the courage to challenge the fears that come with living everyday, and, one by one, overcome them (Chapter 13).