Expansion joints are flexible connectors used in structures like bridges, buildings, and pipelines to accommodate movement caused by temperature changes, settling, or vibrations. They allow for the expansion and contraction of materials, preventing damage such as cracking or buckling. These joints are typically made from materials like rubber, metal, or plastic and are designed to absorb stress. By providing a gap between sections, expansion joints help maintain the integrity of the structure while ensuring safety and longevity. Proper installation and maintenance of expansion joints are crucial for effective performance.