The concept of "no go zones" officially refers to "a town or region barricaded off to civil or military authorities by a force such as a paramilitary, or an area barred to certain individuals or groups", but it's more often cited by racist or islamophobic individuals to stir up hatred and fear – the premise being that areas with a high population of a particular nationality or ethnicity become closed off to the rest of society, including law enforcements.
A cursory google search of the term either comes up with islamophobic books and articles or posts debunking the idea. Still, many people (including Trump) continue to dangerously perpetuate it.
When discussing Canadian "no-go zones", one man on Twitter said:
He was quickly shut down by several different people, one of which was Dan Kaszeta, a security and CRBN specialist.
When he said that we do not have no-go zones in London, the other poster said:
Kaszeta came back with:
When the other man didn't believe him, he quickly posted a photo of his official badge for a committee meeting at Interpol dated that day with his full name printed on it.