Ancient sources have been passed down to us in Medieval manuscripts, which were copied manually. Unfortunately, it is impossible for humans to copy a long text without making errors, which means that our manuscripts are imperfect. Fortunately, copyists often recognized what earlier copyists had done wrong, and corrected this (a “conjecture”): often correctly, sometimes incorrectly. In the end, mistakes proliferated.
The Italian scholar Angelo Poliziano (1454-1494, “Politian”) was the first to understand that the errors were not only a disadvantage but might in fact be used to improve our understanding of the texts. By comparing manuscripts of Cicero’s Letters to Friends, he established that all of them were copies of a book he had seen in Vercelli. This meant that only this manuscript, the “archetype”, had any value; all others could be eliminated. (It is analogous to the elimination of sources.)
In this case, the archetype still existed, but in other cases of course it may be lost. Politian realized that if you looked carefully at the errors, you might reconstruct a lost original. This idea was the beginning of what is called textual criticism.