The Great Barrier is a wall that appears similar to a Gungen shield generator, except in Kekkei Sensen, it's described to be a giant mist that formed. The narrators presume we might know why, or foreshadow why in the anime we should just expect a giant shadow-ey mist to form around a city. Somehow this creates a mystery (that has not been explained in the anime except for Casters.
"Shield" and Relevance. Edit
The anime show explains at the beginning how the president has taken "emergency measures.' Which were not elaborated until episode 2.One prevalent question throughout the series that forshadows the series "Can anyone get through the shield?
Plot device. Edit
For a comparison of other pop-culture references the show references that this mist is another connection to other dimensions and worlds. The plot driven device doesn't explain to the audience how it works, as a 'Maguffin' but more comparable to the plot device of J.R.R Tolkien's Silmarills they exist as a manifesto, a driven device that allows for the show to continue.Leonardo Watch at the beginning is attacked so arguably it's somewhere between a variety of plot devices. IE motivation, functionality etc. The shield Ironically has a similar effect as to in Star Wars.
Further elaboration in episode 9 shows that a C-17 Aircraft was able to fly through the barrier unharmed due to unnatural works surrounding the 'Fog.' However the giant tentacle effect is not explained thus far in the series.