The Bridge-Tool creates a bridge between edges or faces. To use the tool you need to select on both sides the same number of edges or faces.
By default you get something like this:
You can change the default Curve Type Settings to Blend and you will get something like this:
Initially, the bridge will look identical to the “Curve Type: Blend” or “Bridge type: Smooth path”. You need to switch to “Wireframe”-Mode then you can select and adjust the control curve.