Solder bridge is an on PCB conductor with two or more separate pieces that acts as a permanent switch.
In order to short the solder bridge, you should solder between the two parts of the bridge.
In order to disconnect the solder bridge, you should remove the solder bridge by desoldering it.
The solder bridge is used for permanent configuration of the circuit.
You can use jumper or DIP switch for the same functionality. The solder bridge is cheaper, but less easy to use, than jumper or DIP switch.
Solder bridge symbol
The circuit diagram symbol of solder bridge is: