Fundamental Rights
Notwithstanding anything contained in article 13, no law giving effect to the policy of the State towards securing all or any of the principles laid down in Part IV shall be deemed to be void on the ground that it is inconsistent with, or takes away or abridges any of the rights conferred by article 14 or article 19; and no law containing a declaration that it is for giving effect to such policy shall be called in question in any court on the ground that it does not give effect to such policy: Provided that where such law is made by the Legislature of a State, the provisions of this article shall not apply thereto unless such law, having been reserved for the consideration of the President, has received his assent.
This protects laws made to genuinely implement the Directive Principles of State Policy (Part IV) - such as laws aimed at reducing economic inequality - from being struck down solely because they conflict with the equality (Article 14) or freedom (Article 19) rights of some citizens, as long as the law authentically furthers one of those directive principles.
If a state nationalises an industry to better distribute community resources for the common good (implementing the directive principle in Article 39(b)), Article 31C shields that law from being invalidated purely for restricting business owners' Article 19 freedom to carry on business - provided the law genuinely serves that directive principle and isn't just a pretext.