Rename the text in the MultiLayer parameter (at the top of the Parameters tab) from Composite to
MultiLayer.
The MultiLayer node found in the Layer tab accepts an infinite number of input images, each layer containing its own unique settings to control the compositing mode, opacity, and channels. Because you may rearrange the layers via drag and drop in the Parameters tab, it also allows you to work using layer-based, rather than node-based, logic.

Take a look at the Parameters workspace.

Unlike other Layer nodes, the first input represents the background layer, the second input is the next deepest, and so on, until you reach the input farthest to the right, representing the foreground. When the nodes are inserted, they are stacked in the Parameters workspace as well, with the background at the bottom and the foreground at the top. Each layer has associated parameters and controls in the following order:
Parameters | Function |
---|
 | shows the input of the image for that layer in the Viewer |
 | toggles the visibility for that layer |
 | solos the layer, making all other layers invisible |
 | turns off all layers above the current layer and keeps only the current layer and those below it visible |
 | dragging this icon changes the layer order |
 | the name of the input image node; by blanking it out, the node is disconnected, but the layer information remains |
 | the compositing operation for that layer |
 | deletes the current layer |
Try out some of the layer functions: