Why is the dialysis tubing semipermiable?

　　Dialysis, for general use, is semi permeable, like a fish net. A fish net keeps the fish in it, but is permeable to the water, draining the water. A strainer works the same way. Dialysis however blocks larger molecules (which are still relatively small) while letting smaller molecules pass through it, like water. Water is a very small molecule, only three atoms big - H20.
　　That's the general idea. For medical use, as you might be familiar through it in that purpose, is to filter the blood. Wastes in the blood are thankfully made up of much larger molecules than water and everything else good that we want to keep in it. So a dialysis tube filters the blood by filtering larger wastes and bringing back clean blood to the body.

　　For non medical uses, it's to keep larger molecules in it, while letting smaller ones out of it.