Dry and wet

Dry is the signal before the effect. Wet is the effect’s own output. A mix control, often labelled Mix and marked in percent, sets the balance. 0% is the source. 100% is only the effect.

Where you leave it depends on the effect. Reverb and delay are usually well below 100%, because the dry sound is what locates the instrument. A vocal that is only reverb has no note to follow. Distortion, saturation, and filters are often left at 100%, because the processed tone is the one you wanted. If a delay suddenly replaces the instrument, the mix has moved toward wet.

A send splits the same balance across the mixer. The channel stays dry. The send level is how far that channel moves toward the shared wet effect, and the other channels stay where they are.