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.
Marking a closure with @escaping means you have to refer to self explicitly within the closure. Marking a closure with @escaping means you have to refer to self explicitly within the closure.
The content is from The Swift Programming Language (Swift 3.0.1)