Books have been written on this one question and the answer is very different depending on the type of cell, however, in the most general sense the cell wall acts as a selectively permeable barrier between the inside of a cell and the outside of a cell. Essentially it prevents substances going out and coming into a cell unless the cell "wants" those things to come in or go out. It can do this because it is what is called a phospholipid bi-layer, which is just a complex way of saying it has a section that "likes" water and a section that doesnt "like" water. Without going into terms of osmotic gradients, active transport, co-transport, and protien pathways, the cell wall can just let some things, like water, walk in or walk out whenever it wants, but for other things like non water soluble compounds it actually has to "carry" these things across the cell wall.
