request new to c#

Multi-level inheritance simply means that a class can inherit from another class which itself inherits from a third class and so on for as many levels as you like.

So, in this example:

class A

{

}

class B : A

{

}

class C: B

{

}

C inherits from B which itself inherits from A. This is therefore an example of 3 level inheritance.

In fact, to be strictly accurate, it is an example of 4 level inheritance because A inherits 'implicitly' from the System.Object class which sits at the top of the .NET type hierarchy.

You could prevent further inheritance from taking place by making C a 'sealed' class. 

Note that multi-level inheritance is not the same as multiple inheritance. The latter occurs when a class inherits from two or more parent classes. This isn't allowed in .NET which only supports single inheritance i.e. a class can inherit from (at most) one parent.