The Wheatstone bridge is commonly encountered in the application circuits for strain gauges. The bridge is essentially two voltage dividers in parallel with a voltage source. The bridge is balanced, that is when vo is zero, the four resistors obey the simple relation R1R4 = R2R3. [You should be able to demonstrate this with your knowledge of voltage dividers!]
Often the three fixed resistors are selected to be identical in value to the variable resistor R4, the strain gauge, which changes its resistance proportional to the strain. As R4 changes, it unbalances the bridge, thereby generating a voltage vo in relation to the change in resistance.