"Londonistan" is a nickname some people use to talk about London, England. It's a way of saying that there are a lot of people who come from the Middle East and Central Asia who live in London. These people often have different cultures and religions than most people who have lived in London for a long time.
Some people think that these new people are not integrating into British society very well and that there are problems with crime and terrorism related to them. They call the area "Londonistan" to sound like the name of a Middle Eastern country. But it's important to remember that most people who come to London from other countries are just normal people, who are looking for a better life.