The most challenging subject to most people yet interesting to a few and needed in life is math. In my opinion, math concept is different from math skill. A math concept is if a person knows how to perform an operation that he or she can explain it to others. An example is student A learned the concept of PEMDAS and all the rules needed to remember.
He could explain it to others as he has the grasp of the PEMDAS concept. Math skill, on the other hand, is the ability to perform the operation and know various problems that may arise in a given math concept. An example is student A answered a PEMDAS problem with no difficulties and asked his teacher that one equation is not possible as it will not give a proper answer. Usually, for one to obtain a math skill he or she must know the concept first.
Many students find math class difficult. You will see that a student either loves math or they hate it. Some students find it difficult to do. Therefore, they may not know the math concepts, or they are not skilled in math. Someone could see the concept, but they are not skilled enough to accurately perform the concept. They are very similar in definition.
If you understand a math concept like reducing fractions to their lowest term, then you know how to perform that concept when it is given to you by a teacher. However, skill is being able to do that concept if there are problems or errors with it. You may not be able to do the concept because you are not skilled enough at it.