A mosaic is a certain way of making a picture. You take small stones that have different colours. You place the stones in such a way that the colours form a picture. Afterwards, you usually make sure that the stones always stay in place, for example by gluing them down.
There is another way to make a mosaic. You place the stones on a firm cardboard and glue them in place. The picture you make this way is actually a mirror image. Then you fix the whole picture with the free side on a wall. The cardboard is removed. This has a big advantage: you don't have to put the bricks together on the spot, you can do it in the studio. Finally, the gaps between the stones are filled with plaster or a similar compound.
Instead of stones, you can also use other materials, for example glass. Glass can also be used to make windows. Today, ceramics, clay, plastic or many other things are also used.
Mosaics have been around since ancient times. Many other pictures from that time have not survived. But in the case of mosaics, it could be that they were laid out on the floor of an old house. Even if the house was buried, the mosaic remained.