How can reversing current produce an output that keeps one direction?

An alternating supply reverses its voltage repeatedly. A bridge rectifier uses four diodes to direct both halves of that alternation through a load in the same direction. Its output still rises and falls, but its polarity no longer reverses. Turning AC into a smooth, regulated supply requires additional stages.

A diode conducts much more easily in one direction than the other under its normal operating conditions. In a bridge, the diodes connect the two supply terminals to two output terminals by different routes. When one supply terminal is positive relative to the other, one pair of diodes conducts. The route enters the positive side of the load and leaves its negative side.

On the next half-cycle, the supply polarity reverses. The first pair blocks the former route while the other pair conducts. This new route again enters the same positive side of the load. The load therefore receives pulses with one polarity even though the source alternates. The drawing highlights the active route in each half-cycle rather than suggesting that all four diodes conduct at once.

Without filtering, a full-wave rectifier produces two output pulses during each complete input cycle. A capacitor across the output can charge near the peaks and supply the load between them. This reduces the dips but leaves some ripple. The amount depends on load current, capacitance and timing. Regulation can then maintain a more nearly constant voltage despite changes in supply and demand.

Real diodes have a forward voltage drop and limits on current, temperature and reverse voltage. In a bridge, the conducting route passes through two diodes, so those drops matter. A rectifier also does not provide electrical isolation by itself. Its specific job is direction selection. Keeping rectification, smoothing, regulation and isolation separate makes the larger operation of a power supply much easier to understand.