What you see is the sun in the centre of what the creators call an astrolabe, but what should be called an armillary sphere (we thank @Irishpanda for pointing this out):
If I interpret this article (where I also took the concept above from) correctly, the whole map we see is actually on the inner side of a Dyson sphere, and the astrolabe is suspended in its centre.
To quote Angus Wall, the creative director1 of the sequence (emphasis mine):
The fact that I wanted to be able to move the camera anywhere led us to the fact that this whole world had to exist on the inside of a sphere, which took us a while to figure out.
[. . .]
You have a sphere. Next question was “how is it lit?” And obviously, If you have a whole world inside a sphere, what would be in the middle of that sphere? The sun! Or whatever the light source of this world is.
In short, it's a giant sphere around the sun, so the thing you highlight is supposed to be, at least metaphorically, the star that shines upon the world the TV series is set on.
1 Fun fact - same studio made the title sequence to American Gods, the "Origins of Ilvermorny" clip for Pottermore, and a whole bunch of other stuff.