According to WIkipedia...In Native American culture, a dreamcatcher is a handmade object based on a willow hoop, on which is woven a loose net or web. The dreamcatcher is then decorated with personal and sacred items such as feathers and beads.

Traditionally, the Ojibwa construct dreamcatchers by tying sinew strands in a web around a small round or tear-shaped frame of willow.  The resulting "dream-catcher", hung above the bed, is used as a charm to protect sleeping children from nightmares. As dreamcatchers are made of willow and sinew, they are not meant to last forever but are intended to dry out and collapse as the child enters the age of wonderment.

The Ojibwa believe that a dreamcatcher changes a person's dreams. According to Konrad J. Kaweczynski, "Only good dreams would be allowed to filter through… Bad dreams would stay in the net, disappearing with the light of day." Good dreams would pass through and slide down the feathers to the sleeper.

Another version from the same article was, "Nightmares pass through the holes and out of the window. The good dreams are trapped in the web, and then slide down the feathers to the sleeping person."