The gyri are the folds on the brain. The sulci are the grooves on the brain. The gyri and sulci are on the surface of the brain. If you look at a model brain (or a real one, if you choose to), you could see them both. The folding of the sulci and gyri means more cerebral cortex can fit in the skull.
The gyri and sulci have one important job: self-awareness and laughter. With out them, you won't laugh or be aware of yourself. Animals that don’t have gyri and sulci have a less complex brain. Animals with them have a more complex brain.
The gyri and sulci divide the lobes of the brain. There is also a central sulcus which divides the frontal lobe and parietal lobe.