This compensated for number of cases, but not for size of the raw score scale. The scales were weird. So Karl Pearson came up with the idea of first transforming each score to a standard score. So product moment means multiply the deviations This is laborious to convert to z scores, so there are many other algebraiclly equivalent formulae that are easier to do. So as height increases, weight increases a little.
Correlation is low because there are some short fat people and some tall skinny people.
Of course not. That's very high. But can't draw cause and effect conclusions. However, even if you don't know the cause and effect relationship, if two variables are related, they can be used to make predictions. That is, put the x scores on the abscissa, y on ordinate.