A resurgence of nationalism means that nationalistic (very patriotic, maybe hostile towards other countries) feelings, which had been less strong for a while, grew stronger again in the 1820s. So you are supposed to write about the things that could have caused this. (I don't know which country you're writing about but typical causes of nationalistic feeling often include war or the threat of war; immigration on a large scale; social unrest generally.) Hope this helps.