Boxing is to implicitly converting value type to refrence 
type,

int x = 10;
double y = x;


Unboxing is to explicitly converting refrence type to value 
type

double y = 10.234;
int x = (int)y;

The conversion of the value type to reference type is known 
as Boxing and converting reference type back to value type 
is known as Unboxing.

Boxing is the process of converting value type to reference 
type.

Unboxing is the process of converting from a reference type 
to a value type.

Both are slow and should be avoided when possible or 
unneeded.

Dim st AS String
Dim obj As Object
Dim n As Integer = 5
st = n   ' Boxing happens
obj = n  ' Boxing happens

n = st   ' Unboxing happens
n = obj  ' Unboxing happens

BOXING: Converting value type to referenc type.
        Here we go with an example,

ArrayList instance = new ArrayList() 
//In the above line of code,ArrayList is a reference 
type.Add some items to the arraylist. For example,

instance.Add(1);
instance.Add(100);
instance.Add(3);
//3 items added to the arraylist.
//Integer valuee 3 is value type.Adding 3 to the reference 
type(here it's arraylist) is boxing.
i.e., Converting value type to referenc type.

UNBOXING:Converting Reference type to value type.
         Below is an example, retrives the items from the 
arraylist.
         
foreach (int item in arrayList)
   Console.WriteLine(item);

Here arraylist(referenec type) will have 3 values in the 
list, which we added above. 
Adding this arraylist items to an int variable[here int 
variable is "item"] is unboxing.
i.e.,converting the reference type to value type.

Hope you would have understood.
Thanks