A pre-delay can be set (no reverberation until after this period), and a room size. A lowpass cutoff frequency can be used to 'dull' the revereration, and gain controls how much signal is fed back to produce continuing reverberations. Finally the mix of the input dry to processed wet signal can be set.
In this case, the pre-delay is 36mS, the room size is medium and the cut off frequency set to 1103Hz, 66% of the signal is fed back for continuing reverberations and the processed signal is mixed in 5dB below the unprocessed signal.