When creating a scene, rigging a character or once your animation is finished, you can add effects such as blurs, glows, shadows, colour filters and transparency filters and to enhance your project's quality. Effects change the way layers or groups of layers are rendered in your scene.
Effects are special types of layers that you can add to your scene's structure. For an effect layer to work, it must be connected as the child of a drawing, a group, or another effect. Effects only alter the element that they are connected to. This gives you flexibility in deciding which elements of your scene are affected by an effect.
Some effects need to be linked to another layer, referred to as a matte layer, which is used to define the area they should affect.
A matte layer is simply a regular drawing layer that is connected to an effect as its matte. The effect takes the shape of the drawing in the matte layer, ignoring its colours and details, to determine the area of the image to affect.
In the Timeline view, an effect must be rigged as the child of the drawing layer or group it is meant to affect. If the effect can use a matte layer, it will have a Matte parameter when you expand its parameters list. You can drag and drop the layer you want to use a matte for your effect onto the Matte parameter of your effect to link them.
Once your effect is connected, you can adjust its parameters using the Layer Properties view. If desired, you can even animate the parameters of your effect by converting their values to functions.