Effect nodes are special nodes that take an image as their input, and output a transformed image. They are typically connected as the intermediary between a drawing node and a composite.
Some effect nodes have several input ports. The most frequent example is matte input port. Effect nodes with matte input ports need a matte to know which part of the input image to affect.
There is a lot to learn about effect nodes. For more information about how to add, connect and configure effect nodes, see About Effects.