Exactly what ryan said, but to be specific to your last question:
|For example what means L l;?|
1.1) L l is simply creating a object of type "L" that would be called with "l"; a basic class object.
The link above will provide info on polymorphism, but you can think of it as:
Assuming the base as: Shape.
Some polymorphs of shape are: Circle, triangle, and square.
They are all abstracts of shape, having everything shape has plus a bit more; possibly specific to each shape individually.
It can be restrictions, additives, modifications, ect..
In this case, possibly constant number of sides, sum of angles, uniquely available formulas, different child classes, and more...