It's an easy way to make domino lines intersect and a cool effect.
Domino bridges come in domino rally sets or you could make your own one with wood, as shown in DieckDomino's amazing bridge project picture.
Bulk Dominoes has released stand alone Red Bridges found here: RedBridge The red bridges are part of the mega accessory kit. They sell them as part of the Starter, Expert, and Master kits as well. The Classic theme == red bridges. The other themes for these kits have different bridge colors including: Wonder theme == island blue bridges, Power theme == spring green bridges, and Mystery theme == a random chance of clear, black, or brickred bridges.