What is the difference between math content and math skills

The concept is realizing that a common denominator makes adding fractions possible, since the units of the numerator then have the same value.

The skill is knowing how to make it happen-- knowing how to find the LCD, multiplying the top and bottom by the same amount to find the equivalent fractions, knowing that you add the numerators but don't change the denominators, and knowing how to reduce the resulting answer.