Human trafficking is a world issue because it affects countries both rich and poor. This means that it touches practically every country in the world.
Human trafficking affects the poorest countries the most. These are the countries from which the people who are trafficked come. Sometimes, this comes in the form of people signing up to take jobs in richer countries that end up being traps. They are actually being taken by unscrupulous people who will sell them into what is essentially slavery. Sometimes, it comes in the form of people paying to be trafficked. It comes in the form of people paying to be brought illegally to richer countries. Particularly in the former case, this is an issue for the country because it leads to the abuse of their most vulnerable citizens.
Human trafficking also affects the richer countries. Smuggling of human beings leads to greater illegal immigration. It can also lead to some degree of social problems within the richer countries as they come to have pockets of impoverished illegal immigrants living outside the law. It can also lead to situations in which there are people living in these countries who are, for all intents and purposes, slaves.
We can say that human trafficking is a world problem because it affects all countries. We can also say that it is a world issue because it would take efforts by all countries to end it. It would need efforts to improve the economies of poor countries and it would take efforts to reduce the demand for cheap labor in the rich world.