Track envelopes provide a visual representation of the volume, pan position or effect strength within a given track. 

Volume envelopes let you control how volume increases or decreases over time on a track. These can be used, for example, to fade audio in or out.

Pan envelopes let you pan the sound to the left or right.

Effect envelopes let you control the strength of an effect. Effect envelopes can be used, for example, to introduce reverb or an echo at a specific point in the audio track.

To add an effect envelope to a track, you first need to insert an assignable effect into the mixing console. Then, go to "Insert" > "Envelopes" and select the type of envelope you want to add.

You can also use keyboard shortcuts to add envelopes.
Press V to add a volume envelope, and P to add a pan envelope.