A line segment is part of a line consisting of two endpoints and all points between these endpoints. In the figure below, notice how segment AB is part of the line.
The symbol AB can be used to refer to the line segment above with endpoints A and B and it is read as line segment AB or simply segment AB.
Notice also that point A and point B are what we call endpoints.
Basically, if just draw a line between any two points, such as point A and point B, then you have drawn a line segment.
More examples of line segments.