why @escaping closure?

I wold recommend reading this section of the Swift blog post on closures.

The important part is this paragraph:

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns. When you declare a function that takes a closure as one of its parameters, you can write @escaping before the parameter’s type to indicate that the closure is allowed to escape.

So, in the code that Pasan is writing in the video, the closure is escaping because it is called in the callback of the data task. The data task is being returned from the method, but since the task has not been run yet, the data task's callback, and thus the closure, has not been called yet. It will be called after the method returns.

Hope that explains it for you!