In mathematical terms, a tessellation is a tiling of a plane without any gaps or overlaps by a pattern of congruent shapes - shapes that are exactly the same in shape and size. Start with discussing congruent shapes and show examples. It is easy to show how are they are the same size and shape by stacking them in your hand.
Simple ones are squares and equilateral triangles.
Show examples of Escher's work, though he certainly deviated from mathematical concepts. There are commercial sets available of tessellations based on Escher's lizards - they are helpful in demonstrating how tessellations work. Congruent shapes first show up in Texas in the third grade assessment all students must take. Pam Stephens and Jim McNeill have a new children's book out on tessellations that depicts all the steps. Crystal Productions just published it.