Epithelial cells form a layer that line several organs of the body providing a barrier to the outside world. In other words, the epithelial cells help to protect or enclose the organs.
Epithelial cells are held together by tight junctions, which form a virtually impermeable barrier to prevent leakage of fluid (including molecules and ions) across the cell layer, so materials must actually enter the cells in order to pass through the tissue.
Tight Junctions article by Wikipedia
Cell-Cell Adhesion and Communication on Molecular Cell Biology. 4th edition.
Cell Junctions on Molecular Biology of the Cell. 4th edition.