Also check Samples and Track settings

Here you see the envelope details for the selected (pink) sample.

Basicly an envelope has two nodes : begin and end node. You can however insert additional nodes (as shown here).

For every node you can see the exact position and volume. The envelope is also represented by a line + nodes drawn on the sample.

Add: adds a node, before the selected node

Insert: inserts a node, after the selected node

To add nodes directly on the sample, you can hot CTRL + left mousclick

Delete: deletes the selected node

To delete nodes directly on the sample, hit CTRL + right mouseclick

Link Volume Changes: link all nodes together when you change the volume. This means, changing the volume while this option is set, will change the volume equally for all nodes on this sample.

On the grid, you can change nodes by click and dragging them.