There are two different definitions of penetrating DR… so it really depends on what exactly you are looking at that brings up the question…
Generally, doing enough damage to still inflict actual damage is in-fact penetrating DR.
There are also some special abilities that allow you to ignore some or all of a creatures DR, such abilities would also be considered penetrating DR.
For most effects that call out penetrating DR they refer to the former. But there are some specific abilities that do refer to the latter, though when that is the case it should be rather obvious, typically by calling out an ability that does so… generally such abilities are referred to as ignoring or bypassing DR though leaving penetrating DR to largly refer to dealing enough damage to inflict actual damage after applying DR.
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It is a good idea to cite the relevant feat in the question, not only in the title.
Rebuffing Reduction (Combat)
Source Ultimate Combat pg. 115
Your damage reduction can turn the force of blows back on your enemies.Prerequisites: Str 13, damage reduction, Improved Bull Rush, Power Attack, base attack bonus +1.
Benefit: Whenever an opponent that is adjacent to you fails to penetrate your DR with a melee attack, you can spend an immediate action to attempt a bull rush combat maneuver against that opponent. If you succeed, you cannot move with the opponent.
In this instance, as Chell Raighn said, failing to penetrate DR means that the DR negates all the damage dealt.
Damage Reduction
To avoid possible misunderstandings, the ability to negate a target DR is called "overcoming" it, not penetrating it.