What do F and D mean at the end of numeric literals?

Means that these numbers are doubles and floats, respectively. Assume you have

void foo(int x);
void foo(float x);
void foo(double x);

and then you call

foo(5)

the compiler might be stumped. That's why you can say 5, 5f, or 5.0 to specify the type.

D stands for double

F for float

you can read up on the basic primitive types of java here

http://download.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

I would like to point out that writing

5.1D or 5.1 : if you don't specify a type letter for a comma number then by default it is double

5 : without the period, by default it is an int