Find the misunderstood words that are blocking comprehension, get those words and concepts resolved and understood, and don’t move past them if they aren’t. Math and science, when taught competently, build on blocks of general and fairly simple concepts. Yes, it’s possible to go off the rails if you try to learn too much, too fast, and without any real understanding of the basic concepts, but that’s not what usually happens.
It seems to me that what happens is a failure to grasp the fundamental concepts, or a rote memorization of “do this problem this way” takes over, so that the student can’t really “understand” what is being taught. They try to become computing machines to “solve problems” without a basic understanding of how mathematical (or scientific) basics apply.
So, yes, dropping the current class if one is in this position early in the school year might be appropriate, but it would also be appropriate in that case to repeat the pre-requisite course/s that lead up to this one.