A line segment is a part of a line that has two endpoints. It is defined by these endpoints, which can be labeled with letters, such as A and B. The length of a line segment is the distance between these two points, and it can be measured using a ruler. Line segments are fundamental in geometry and are used to form shapes like triangles and rectangles. They can also be represented in a coordinate system, where the endpoints have specific coordinates, such as A(1, 2) and B(4, 6). This helps in calculating their length and understanding their position in space.