Bridges in 3D printing are when you print two connection points of a model that have little or no support between them. Normally, a 3D printer first prints the base layer, on which it builds up the object layer by layer. In contrast, there is no base layer between two points in bridging. The problem with bridges is that they can sag or hang instead of forming straight lines. This is because the tension holding up the bridge is not enough to overcome gravity. This can lead to poor print quality, stringing or even failed prints.