Precomposing is a way to nest layers within a composition. Precomposing moves the layers to a new composition, which takes the place of the selected layers. When you want to change the order in which layer components are rendered, precomposing is a quick way to create intermediate levels of nesting in an existing hierarchy.
After you apply an effect to a layer in a composition, you can customize its properties in the Effect Controls panel. This panel opens automatically when you apply the effect, or you can open it at any time by selecting the layer with the effect and choosing Window > Effect Controls.
To modify the transparency of a layer, reduce its opacity. Select the layer in the Timeline panel, press T to reveal its Opacity property, and enter a value lower than 100%.