In the semipermeable lab experiment, certain solutions are prepared and they will be held through a semi permeable membrane in order to determine if it will diffuse or not. At the end of the experiment we will notice that starch solution only permits water to pass through the membrane but not the starch. NaCl solution on the other had will diffuse water and the salt. 

What causes this diffusion?

Selectively permeable membrane only allows the passage of smaller molecules. Larger molecules do not fit in the membranes. Starch are large molecules of carbohydrates where as NaCl are smaller molecules.

NaCl will form ions in the solution.

NaCl ---> Na+  and Cl- ions that are generally small to pass through the membrane.

Starch is composed of large number of glucose units joined together by glycosidic bonds. A single glucose unit has a formula of C6H12O6 which is a lot bigger than Na+ and Cl- ions. What more if it is a group of glucose units?

Hope this helps :)