A Mood Board (or Visual Territory Board) is a design deliverable consisting of visual elements that capture the high level style of a design direction. It is basically a collage of (existing) images and text in a composition. Created in the early stage of a project they help you setup the direction for the rest of the design. Also they help you review visual ideas with the team or client to agree on the initial direction. Mostly they are followed up with a Style Tile or Concept Board and are used as reference point.
I made three mood boards to show you which directions we can go