A tessellation occurs when a shape is repeated over and over again covering a plane without any gaps or overlaps.
The shape is "repeated" by the basic transformations of translations and/or reflections and/or rotations.
The regular hexagon above can form the tessellation by translations, reflections or rotations.
Not all shapes form tessellations.
The scalene triangle below forms a tessellation
The "arrow" shape below forms a tessellation
The composite figure below forms a tessellation
The regular pentagon shape below DOES NOT form a tessellation. Whenever gaps or overlaps are not preventable, a tessellation cannot be formed.