A defensive line is a defended position, made to hold the front of battle or maintain station in the midst of an offensive.
Generally, a defensive line is comprised of defensive structures and mobile units... with this combination it can be quite effective. However, a failure to produce both for such a use generally leads to defeat or inefficency in a balanced mod.
Many new players invest themselves in a costly defensive line, which commonly goes hand in hand with "porcing". However, most experienced players use an equivalent to help hold territory.
Defensive lines are generally effective to some degree, and remain a viable tactical development in some situations throughout any balanced mod.