The left-hand graph ("W-space") represents three weights that add up to one. The shaded region corresponds to weights between zero and one.
The right-hand graph ("P-space") represents the volatility (x-axis) and mean return (y-axis) of the portfolio with the given set of weights in W-space.

First click the two check boxes in the right-hand graph so that there are no curves visible. Move the point W in the left-hand graph and see how the point P moves in response.
Now click on the "frontier" check box. What does the line in W-space and the curve in P-space represent?
Finally click on the "boundary" check box. What do these curves in P-space ("Markowitz bullets") correspond to?