Before you proceed to learn the racing line, I'd like to recommended to first start with reading the basics of racing. You should also have a fundamental understanding of the basic differences between oversteer and understeer.
What Is The Line?
Simply put it's the fastest route (line) to clear a corner. Its the best way to enter the corner, clear the apex, and then exit the corner with the highest possible exit speed.
On the racetrack you can usually identify the racing line by the trail of rubber left behind by previous drivers.
The tarmac is darker on the racing line, especially in the braking and cornering zones.
Typically youll see the racing line starting on the outside of the track, near the edge. Then the line cuts into the corner, hitting the inside point (clipping point), after which it goes towards the corner exit, again completely on the outside.
Of course each corner is different, and each corner has its own best racing line. It is up to the driver to discover and learn this line- exactly why track knowledge is just as important as car control.