Parameter can be used in two different ways: As method parameters and as class parameters.

Method Parameters

Method Parameters are used in every method and constructor declaration:
func add(l: int, r: int) -> int = r + l
^ ^
In the above example, l and r are parameters of the add method. Both parameters are of type int. When calling the add method, you will have to pass two arguments that are either ints or can be implicitly converted to int.
let i1 = add(1, 2) // 3
let i2 = add(2, i1) // 5
let i3 = add(i1, i2) // 8
let s: short = -1
let b: byte = 2
// s and b can be converted to int
let i4 = add(s, b) // 1

Class Parameters

Class Parameters are similar Method Parameters, but declared on classes.
class IntAndString(i: int, s: String)
By annotating a class with parameters, you can force the compiler to generate fields and a constructor. The above Dyvil class definition would look like this in Java:
class IntAndString
int i;
String s;
public IntAndString(int i, String s)
this.i = i;
this.s = s;
As you can see, using class parameters saves you a lot of boilerplate code. Still, an instance of the IntAndString class has to created like this:
let ias = new IntAndString(1, "a")
Class Parameters are even more useful in Case Classes.