From Photoshop Help files:
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Multiply
Looks at the color information in each channel and multiplies the base color by the blend color. The result color is always a darker color. Multiplying any color with black produces black. Multiplying any color with white leaves the color unchanged. When you’re painting with a color other than black or white, successive strokes with a painting tool produce progressively darker colors. The effect is similar to drawing on the image with multiple magic markers.
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In printing terms Multiply is the same as Overprint, as Normal is the same as Knockout. If you had a white object or type overprinting on a colored background, it would not print.
