Many surfaces of an animal body, and many linings inside an animal's body, consist of sheets of cells that act as highly selective barriers to the passage of materials, in either direction. These cell sheets separate the two sides of the organ or structure (the "inside" and the "outside"), and usually these two "sides" have very different chemical compositions, that must be kept separate and apart.
For example, the epithelial cells that line the small intestine in humans are arranged into a sheet of cells that separate the contents of the guts and the inner cavity of the organ that eventually empties into blood vessels.
While acting as a barrier that prevents the semi-digested food from mixing into the bodily fluids, this sheet of epithelial cells must also act as a very selective pump, pumping required nutrients (such as glucose) from the digesting food and into the blood.
Special tight junctions between the cells of the epithelium are very important in helping the cells stay together as a sheet of cells (i.e. joining them to one another) and also helping the cells act as a very selective barrier.
Continuous strands of "junctional proteins" form a weld between the plasma membranes of two adjacent epithelial cells in such a way as to make a very tight contact between the two sets of plasma membranes, cross the intercellular space, and then seal off the spaces between them totally and completely.
These strands of sealing proteins extend all the way around the cell forming a complete circle. At such junctions the lattice-work of junctional protein strands unites the two sets of membranes so closely that there is no intercellular space, or gap, between them.
When nutrient molecules, such as glucose, are absorbed by these cells on their outer surface (that in contact with the digesting food in the gut), they past through the epithelial cells and are then passed out of the cell once more into the blood. Tight junctions between the epithelial cells prevent the glucose molecules from diffusing around and between the adjacent cells, prevent transport proteins from moving from one surface of the cell to another, and unite the epithelial cells into a continuous sheet that prevents the gut contents from leaking into the blood.