I think the answers others have given you definitely apply to the very far right voters. They are against all form of immigration. But that doesn't explain the AFD's rise in popularity. So I'm going to talk about the more moderate rightwing people here. There is a lot of immigration into Germany, EU migrants and legal, as well as illigal immigration from outside the EU. Plus Ukrainian refugees. Most people on the right don't mind EU migrants as well as Ukrainian refugees. Ukrainian refugees btw. are mostly women and children. Legal migrants from outside the EU are more complicated, and I'm sure there are various levels of acceptance. The real problem though are illegal immigrants, mostly from the middle east and Africa. They are mostly men with poor education and in many areas the infrastructure to accommodate them are at their limits. It doesn't help that these places are neglected and violence does break out at some of them every now and then. These stories obviously spread on social media like wildfire.