Blending modes are one of the fundamental features of Photoshop: they allow you to combine layers in varying fashions. It can be a bit of an experimentation game working with them, though. This comprehensive video will show you exactly how each blending mode operates and allow you to take more control of your work.
Blending modes, at their core, are mathematical equations that tell Photoshop how to take luminance (and sometimes other) values and combine them. Photoshop normalizes luminance values on a scale of 0 (black) to 1 (white). It then uses the values of each channel to combine layers according to the equation associated with the blending mode. For example, suppose we have a base layer pixel of (204, 120, 8), which looks like a sweet potato orange, and an active layer pixel of (100, 55, 160), a semi-deep purple. Normalizing these gives (.8, .47, .03) and (.39, .22, .63). Using the Multiply blending mode, we multiply each channel value with the corresponding channel value in the other layer, giving (.31, .1, .02), a reddish brown. Since all the values are always between 0 and 1, multiplying them generally shrinks them, which is why the Multiply mode darkens the image.
Don't worry, though; if you'd rather not get into the math of blending modes, you can still gain an intuitive grasp of how they work, which will allow you to harness their power much more. Give the great video from Jesús Ramirez a watch to learn about them!
