The model origin appears in blue and represents the (0,0,0) coordinate of the model. When a sketch is active, a sketch origin appears in red and represents the (0,0,0) coordinate of the sketch.
Dimensions and relations can be added to the model origin, but not to a sketch origin.
To toggle the origin display:
Click . When the icon next to the menu item is highlighted, origins are visible (except for origins you have hidden individually).