This is the portion of a line where you have a point on one end to show that the line stops in one direction but in the opposite direction an arrowhead is placed at the end of the line to indicate that the line goes on indefinitely.

Geometry has three building blocks: points, lines and planes.  Everything else in geometry will build off of these three blocks.

A point is a single location in space.  Wherever two lines intersect one another, that exact place where the lines intersect is known as a point.  Through any two points there is exactly one line.
A line is an indication of distance that continues indefinitely in two different directions and is always straight – it is never curved.
A plane is any flat surface such as a floor or a wall or a piece of paper.  The intersection of two planes creates a line.