TreeofLife and incognito said it all. To expand, I would basically outline how my research contributes to the main subject and the problems its trying to address at the macro level...obviously big parts of your research would be beyond most undergrads, but would certainly feed their appetite for the subject in general.
I'd also have a look at the department profile and using that, discuss how your research adds to the intellectual / academic capacity of the department and perhaps provide opportunities to specialize in key areas of the subject not currently not taught in the department. (i.e. possibly attracting wider enrollment)
Just my 2c