There are many different layer modes that Gimp offers. Most of them are quite similar to Photoshop’s layer modes. I’m going to break each one down for you and explain each one just like the Gimp Manual.

1. Normal Mode- This is the default layer mode. The layer on top covers the layers below it. If you want to see anything below the top layer when you use this mode, the layer must have some transparent areas.
2. Dissolve Mode- This mode dissolves the upper layer into the layer beneath it by drawing a random pattern of pixels in areas of partial transparency. It is useful as a layer mode, but it is also often useful as a painting mode.
3. Multiply Mode- This mode multiplies the pixel values of the upper layer with those of the layer below it and then divides the result by 255. The result is usually a darker image. If either layer is white, the resulting image is the same as the other layer (1 * I = I). If either layer is black, the resulting image is completely black (0 * I = 0).
4. Divide Mode- This mode multiplies each pixel value in the lower layer by 256 and then divides that by the corresponding pixel value of the upper layer plus one. (Adding one to the denominator avoids dividing by zero.) The resulting image is often lighter, and sometimes looks “burned out”.
5. Screen Mode- This mode inverts the values of each of the visible pixels in the two layers of the image. (That is, it subtracts each of them from 255.) Then it multiplies them together, divides by 255 and inverts this value again. The resulting image is usually brighter, and sometimes “washed out” in appearance. The exceptions to this are a black layer, which does not change the other layer, and a white layer, which results in a white image. Darker colors in the image appear to be more transparent.
6. Overlay Mode- This mode inverts the pixel value of the lower layer, multiplies it by two times the pixel value of the upper layer, adds that to the original pixel value of the lower layer, divides by 255, and then multiplies by the pixel value of the original lower layer and divides by 255 again. It darkens the image, but not as much as with “Multiply” mode.
7. Dodge Mode- This mode multiplies the pixel value of the lower layer by 256, then divides that by the inverse of the pixel value of the top layer. The resulting image is usually lighter, but some colors may be inverted.
8. Burn Mode- This mode inverts the pixel value of the lower layer, multiplies it by 256, divides that by one plus the pixel value of the upper layer, then inverts the result. It tends to make the image darker, somewhat similar to “Multiply” mode.
9. Hard Light Mode- This mode is rather complicated because the equation consists of two parts, one for darker colors and one for brighter colors. If the pi