Tight junctions are formed between adjacent cells of the epithelia. The epithelial cells form a layer between a cavity and a mass of cells. For example the lungs have an epithelia that acts as a layer between the mass of cells that make up the lungs and the cavity filled with air.
The tight junctions do not allow the passage of substances, through gaps between cells, into a cavity. For a molecule to reach the cavity it is essential that it enters the cells and then passes to the other side through it. The tight gaps also block the free passage of integral membrane proteins between the portion of the cell exposed to the cavity and the rest of the cell.
An example of tight gaps is that between the epithelial cells of the lungs. Here, the tight gaps prevent the passage of the growth stimulant heregulin that lies in the apical surface to the basolateral receptors that are sensitive to it.