Texture synthesis is when you take a picture or pattern and use it to create a bigger picture or pattern that looks similar or almost the same. It's like making a puzzle out of different puzzle pieces, but instead of putting the pieces together, you make a new puzzle piece that looks like the original ones.
For example, imagine you have a picture of a brick wall. You can take a small section of the wall, maybe just one brick, and use it to create a bigger picture of the wall. The computer will take that one brick and figure out how to put it together with other bricks to create a new, larger section of the wall that looks like the original.
Texture synthesis can be useful in many ways. If you're a designer, it can help you create patterns that look more natural or realistic. If you're working on a video game, it can help you generate backgrounds or landscapes that look more detailed and less repetitive.
Overall, texture synthesis is a complicated thing that computers do to make pictures and patterns look nice and more interesting.