وَإِنْ أَصَابَتْهُ نَجَاسَةٌ مُخَفَّفَةٌ، كَبَوْلِ مَا يُؤْكَلُ لَحْمُهُ: جَازَتِ الصَّلَاةُ مَعَهُ مَا لَمْ تَبْلُغْ رُبُعَ الثَّوْبِ.
Translation: If light impurity, such as the urine of an edible animal, reaches a garment, prayer is permissible so long as it does not reach one quarter of the garment.
Light impurity is measured against one quarter of the garment. The phrase “so long as it does not reach” means the concession is limited to amounts below that threshold.