what does it do ?

hit { to start building a list.
type here your program.

The program is 5 DUP 3 + 2 ->List

what does it do ?
5 enter 5 in the stack
DUP dupplicate it, you have now 5 and 5 in the stack
3 enter 3 in the stack, you have now 5 , 5 and 3 in the stack
+ perform addition (3+5), you have now 5 , 8 in the stack
2 enter 2 in the stack, you have now 5 , 8 and 2 in the stack
->list create a list (of 2 items), you have now { 5 8 } in the stack

Finaly this program just perform some operation and put a dummy list in the stack.
All these operation does not have any sens, it's just for an example

May you should take a look at Tutorial 1, RPN Basis to know how to enter number into stack...