The valence band is a crucial concept in solid-state physics and materials science. It refers to the range of energy levels in a solid where electrons are present and can participate in bonding. In semiconductors and insulators, the valence band is filled with electrons, which are responsible for the material's electrical properties. When energy is supplied, such as through heat or light, electrons can be excited from the valence band to the conduction band, allowing them to move freely and conduct electricity. The gap between these two bands is known as the band gap, which determines the electrical conductivity of materials like silicon and germanium.