Ibn al-Qayyim said: "The scholars are oceans, and their mistakes are like impurities - and water, when it amounts to 2 qullas (a measure of volume), does not carry impurity."
Hafiz ibn al-Qayyim (ra) is alluding to a hadith narrated by Ibn Umar that the Prophet –sallallahu alayhi wa sallam- said: “ If the volume of water amounts to 2 qullas, it does not carry impurity.”
Based on this hadith, many 'ulama opine that a large quantity of water (i.e. more than qullatayn) is not rendered impure by coming in contact with impure substances as long as one of its attributes (taste, color, or smell) is not changed.
People often tend to speak ill of our imams by mentioned their faults. Ibn al-Qayyim is saying that just as some impure substance in an ocean (I assume the water in an ocean would amount to millions of qullahs) is negligible and does not render it impure, the flaws of our imams are negligible in light of their merits and do not detract from their lofty status in the least.
To give more context, I came across this quote in an article defending Imam Abu Hanifah against those who make mention of some of the statements of the ashab al-hadith against him to detract from his status.