It is only when there is not even the least vestige of ignorance left in your heart that you will be capable of understanding wisdom. Ignorance is like fire. Suppose a fire is extinguished almost completely, except for just a few glowing embers. If a breeze comes up, sparks from these few coals may develop into a huge conflagration. Therefore, there should not even be a remnant of the fire left.
Ignorance is also like sickness.
If, after coming home from the hospital, you give up the proper diet, it may quickly develop and spread again. There should be absolutely no remnant of disease left. You can also compare ignorance to being in debt. The debt will start piling up again. Therefore, you should discharge your debts completely.