The tile in a bathroom or kitchen is one example of tessellation. It's where you place geometric shapes together to fill a space. In this case, we have squares placed together to fill the space of the wall or floor. There cannot be any gaps or overlaps.
Another example would be bricks as this is composed of rectangles placed together without any gaps or overlaps. There are many other examples as well usually done as floor or wall patterns of some kind. Some tessellations involve using more than one shape (eg: one pattern uses squares and equilateral triangles placed together).