My understanding--and admittedly it is a limited understanding--is that the reason, on a physical level--literally on the level of physics--that people can't walk through walls (or that any solid objects can't pass through each other) is because of the negatively charged electrons that spin around the outside of the atoms that make up . . . everything. Thus, when you bump into a wall, what's really happening is the electrons in you are banging into the electrons of the wall, and the "like" charges repel. This, if accurately described, is elementary physics.
What I find interesting is that, if this description IS true, it suggests that--in what would surely be a one in a googolplex occurence--if all of a person's atoms lined up JUST right in relation to the atoms of a wall, then that person could, indeed, walk right through said wall. Which I think would be really cool.
That is all.