There are 12 pairs of results, so there are 12-2=10 degrees of freedom. Using a=0.05 we see from the critical values table that the critical value of r is 0.576, but, since r=0.780, which is bigger than the critical value, there appears to be a correlation, and so the null hypothesis (that the results are due to chance) isn't valid (with a 95% chance of being correct about this). Therefore there appears to be a significant relationship to body weight and jump height (the alternative hypothesis).
Using the formula t=rsqrt((n-2)/(1-r^2))=0.780sqrt(10/(1-0.6084))=0.780sqrt(10/0.3916)=3.94. This value is higher than the 2.23 value in the t-test values table for a=0.05 and 10 degrees of freedom, so reinforces the hypothesis that there is a significant relationship between body weight and jump height.