Suppose that a biologist states that the average height of undergraduate students at your university is 205 cm plus or minus a standard deviation of 17cm. What does this mean?

This means that the means height (which is all the sum of the height of all the students divided by their number) is 205 cm. Standard deviation means how the height of the students deviates from this mean since students do vary in height and are not all the same height of 205 cm. Therefore, in this case, the amount of standard dispersion of the students' height is by 17 cm on either lower or higher side of the mean.