Exactly... hair thickness refers to hair density, and strand thickness refers to the individual strand size.
One way to see if your hair is thick, is to look at a 1in square of your hair. If you grip that 1in square together and pull taught to mimic straightness, is the circumference the size of a dime, nickle, or a quarter? Average or medium thickness would be about the same circumference of a nickle.
HTH!