Legal Maxims refer to a body of abstract rules that were produced after a detailed study of fiqh (Islamic jurisprudence). They are theoretical guidelines corresponding to different areas of Islamic Jurisprudence. The above maxim is on the chapter of purification dealing with the types of impurities (Najaasa), and how they are cleansed.
The maxim classifies impurities into two categories:
1-Natural impurity: they are the natural impurity that cannot be separated from it in any circumstance. Examples of such impurities are urine and stools of humans, and self-impure animals such as pig dog etc. the dog and the pig are created filthy and impure. Their filthy composition is part of their initial and original creation. According to the maxim, the filth of these impurities cannot be purified in any way even if washed and cleansed with an ocean of water.
The process of tanning cannot purify the skin of the pig or dog, even though scholars differed concerning purity of the skin of the pig after tanning. The most preponderant view on this is that the skin of the pig remains filthy even after tanning because its impurity is a natural one, unlike the skin of dead eatable animals such as goat, sheep and cow.
2-presumptive impurity: they are incidental impurities that occur to a pure place or object. As for this, anytime the filthy element is taken out of the pure place or object, then its ruling (of impurity) is taken away (and the place or object becomes as before-pure-). Scenarios that fall under this category are many for example: if a piece of cloth or dress is affected by filth such as urine and is washed out, the dress becomes pure and can be used to offer salaat. The same applies to a shoe or sandals and anything that is pure but was touched with filth and an impure element.