The icon becomes active if at least one part is selected. Click the icon to carry out the nesting for the selected parts.
You can define the nesting rules in Machining... > Fixtures.  

In the context menu, you can perform local nesting in addition to global nesting.

Nest parts in blank – Local area

The part is optimally placed in the blank near the current position.

Nest parts in blank – Global area

The part is optimally placed in the blank according to the specifications in the fixture settings.