Your work should be put into context. A clear description of the topics you have worked on and the questions you have tried to answer is essential. Writing the thesis gives you one opportunity to demonstrate that you do know something more than your own results and their proofs. It is usually a good idea to write a plan setting out the expected chapters of the thesis and giving a very brief outline of the contents of each chapter. Normally you would discuss your plan with your supervisor and when agreement has been reached, you proceed with the writing.
Every thesis has an introduction which tells the reader what it is about.
It will briefly summarise the problems you have worked on, explain how these problems arose from existing work and perhaps give an overview of the main results of the thesis. An introduction often finishes with a description of the contents of each chapter. Although it comes first, the introduction is usually written last. The introduction is followed by a chapter giving background information. Remember that the thesis will be read by people who have not been immersed for two years in your research problems.