The "band gap" is the energy difference between the highest energy electrons in a material and the lowest energy states that are unoccupied. It determines how well a material can conduct electricity. Materials with a small band gap can easily conduct electricity, while those with a large band gap are insulators. In semiconductors like silicon, the band gap is moderate, allowing them to conduct electricity under certain conditions, such as when exposed to light or heat. This property is crucial for the operation of electronic devices, including transistors and solar cells.