What is the point of a thesis defense presentation?
The point of the thesis presentation is so the examiners can see if you actually did the work in the thesis or not.
It's quite easy to paraphrase content from e.g. publications, to ask one's supervisor for assistance tackling technical aspects of the work. So, by seeing you present the work and asking you some questions about the content, the examiners can assess whether you actually understand what you're talking/writing about.
Think of it as a presentation followed by an oral examination on the content. Some tips (I am not a mathematician but hopefully these are generally applicable):

Be clear about what problem your work is addressing.
Be clear about why that problem is actually worth addressing.
Make sure you give due credit to the relevant literature (e.g. with citations).
Don't spend more than 1/3 of the presentation on the points above.
Focus on what your contributions were to solving the problem described above.

Hope this helps.