An effect always needs a drawing connection and sometimes a matte or shape connection. A matte provides drawing information that determines the area on which the effect will be applied on the drawing. The details and colours within the matte drawing don't matter, as only the shape and transparencies are used. A matte is also known as mask.
The matte principle can be demonstrated with the Cutter effect.
In general, a drawing layer is connected to the right port of the effect and the matte (mask) to the left.
The Cutter effect cuts out a portion of an image using a matte drawing. You can use the Cutter effect to allow a character to disappear behind a background element or to cut a virtual hole into the layer itself. The Cutter requires a matte input.
Adjust the Cutter properties to invert the effect of the matte on an image. Instead of the object being hidden by the mask, it will only be visible where it intersects with the matte shape area.
You can adjust the effect parameters in the Layer Properties view and link any of them to a function column to be animated over time. This means that all of the effects can be customized.
If, in addition to having one layer cut by another layer, you still want to see the cut layer, drag another connection from that drawing node's port and connect it to the composite.