Create an envelope, spanning the entire project duration.
For example, this Node can be used to fade phases in a song in and out, by connecting to a Gain Node‘s gain control input.
The envelope can be edited on the Timeline. To enter edit mode, right click on the Envelope Clip, then choose Edit (for touch screens: tap the Envelope Clip, then tap on the edit button that appears over it).
See Envelope Editor for more information.