Blending modes for layers control how each layer blends with or interacts with layers beneath it. Blending modes for layers in After Effects (formerly referred to as layer modes) are identical to blending modes in Adobe Photoshop.
Most blending modes modify only color values of the source layer, not the alpha channel. The Alpha Add blending mode affects the alpha channel of the source layer, and the silhouette and stencil blending modes affect the alpha channels of layers beneath them.
You can’t directly animate blending modes by using keyframes. To change a blending mode at a certain time, split the layer at that time and apply the new blending mode to the part of the layer that continues. You can also use the Compound Arithmetic effect, the results of which are similar to those for blending modes but can change over time.
You apply a blending mode to the layer above the layers you want it to interact with.
Blending modes for multiple masks on a single layer are called mask modes.
Some effects include their own blending mode options. For details, see the descriptions of the individual effects.