Abstract class derived from a specific class

I just got a warning in my IDE that my class is abstract, but it is derived from the class concrete. So what? Why can't I do this? I love it. This is just a warning, although the class is compiling. Just wondering why this gives me this warning. Thank.

Update

I need to clarify some questions. I have this class derived from another class, and I want no one to be able to create it. Moreover, I need two subclasses from this abstract class, and each of them must have its own implementation of one abstract method. That's why.

Do you think this is a bad idea? Which approach will I take instead?