What is clonal selection theory and why it is so important?

Clonal selection theory is a scientific theory in immunology that explains the functions of cells (lymphocytes) of the immune system in response to specific antigens invading the body.

The concept was introduced by an Australian doctor Frank Macfarlane Burnet in 1957 in an attempt to explain the formation of a diversity of antibodies during initiation of the immune response.

Just search about the terms you're not familiar.

Source:

http://en.wikipedia.org/wiki/Clonal_selection