Positive half-cycle: the diagonal pair of diodes conducts and current flows through the 1kΩ resistor in one direction.
Negative half-cycle: the opposite diagonal pair conducts.
Even though the AC source reverses polarity, the current through the load resistor remains in the same direction.
Therefore both halves of the AC waveform appear as positive pulses at the output.
The green waveform at the top is the full-wave rectified output (pulsating DC).
This is why the output frequency is twice the input frequency and why a full-wave rectifier is more efficient than a half-wave rectifier.