The SEP entry "Rationalism vs Empiricism" distinguishes between the terms concept and knowledge.
Is there some standard distinction between these two terms that's commonly used by most philosophers? (And is it being used here by this writer?)
Short answer. Traditionally, the genus " cognition" ( "cognitio") is divided into species : "apprehension" ( grasping a concept) and judgment ( combining concepts) . Knowledge belongs to the second species ( to know is to judge with certainty).
Note : reasoning was reduced to judgment ( mediate judgment).
The context of your question is rationalist inneism, the idea that human soul is not originally a " tabula rasa" ( contrary to what empiricists contend).
The point is to distinguish concept inneism and knowledge inneism.
Knowledge inneism contends that, as soon as it begins to exist, the human mind already has a certain stock of truths in itself, that is propositionnaly structured states of mind. This is Plato's thesis acccordng to which leaning is nohing else but recollection ( " anamnesis").
Now, this thesis is difficult to acccept, because it implies that a three months baby knows unconsciously that , say, the square of the hypothenuse is equal to the sum of the square of the two other sides of the triangle. When this baby has grown up, he will come across a demonstration of this theorem; and he will recognize this truth; the truth will not be infused into his mind (by the book he reads of the geometry teacher).
Now, if one wants to escape the difficulties of this thesis, one can say that there is , in human mind, some innate cognition that is not propositionally structured. This innate cognition provides only the elements of propositional knowledge and consists in " concepts" or " ideas" .
An analogy : a three months baby does not know innately how to walk, but the elementary physiological abilities ( reflexes, muscular connections, equilibrium sensitivity) that he will put in use when he learns how to walk are already there at his birth .
According to Leibniz, innate ideas are natural dispositions to grasp fundamental essences or categories such as " substance" or " number( collection of unites) " or " cause/ effect relation". Because the human soul is by nature a substance, because it is one being ( a unity) , because it has an acting will producing effects, and because it is naturally able to reflect on itself, these ideas do not come from outside; they are here in the same way as words already written on the wax of the soul.