Any robotics project might involve running a motor. Since the power requirements of a motor is much larger than a micro controller, we cannot use the output pins of a microcontroller to run motors. This would heat up the MC and damage it.
So traditionally transistors, mosfets, relays are used to drive motors which acts as switch i.e using a small output current from a MC, driver a larger current for the motor.
Now for a DC motor, if apply +ve voltage to one end and -ve to another, it run in one direction. If we change the polarity, it runs in another direction. H Bridge is basically a circuit to run a motor in both directions by changing polarity of voltage.