For the introduction to a history master's thesis, you're looking at including a few key things:
* the question(s)/topic under investigation - set out clearly what the thesis will investigate;
* some historical context. You don't need to give a blow-by-blow account of the events/people/phenomenon you're exploring, but some basic context is useful. Bear in mind that the examiners will probably know a bit about the topic, but may not be versed in it in great detail, so providing some background is helpful;
* a historiography review - outlining what has been written on the topic before, and how your work fits into that;
* an outline of your sources, and how you've used them. This could be described as 'methodology', though many historians struggle with that. Just explain your main sources, their strengths and weaknesses, etc.;
* a brief outline of the chapters, and what each will cover.
One thing to try and weave throughout the intro is an answer to the 'so what?' question. Why have you chosen to write on this, and why is it important or relevant? You can answer that at various moments, such as when outlining your research questions - spell out how your approach or focus is relevant and/or original. Likewise in sections on the historiography and sources, there are also opportunities to show why your particular topic needed to be done/expands existing knowledge.
Hope that helps! The intro is often the hardest part to write, so be prepared to give it the time it deserves, and to do rewrites etc. It is important to nail the intro: it's a cliché but a poor or confusing introduction can really put off readers, and leave them less willing to really engage with the rest of the work, so spend the time on it!