Stances determine how actively your units respond to enemy units in their <link="topic:line of sight">line of sight</link>. There are three stances:

Aggressive:  Your units will chase down and fight enemy units in <link="topic:line of sight">line of sight</link>.
Defensive: Your units will fight enemy units within <link="topic:line of sight">line of sight</link>, but will not stray far from their last commanded location.
Passive: Your units will neither fight nor flee.

Stances may be set on individual units or groups of units.