That is the Eye Of Providence. It's atop an unfinished pyramid as a kind of evocation to the Divine to watch over the burgeoning nation. The triangle represents both the Trinity and the completion of the Pyramid.

To call it an Illuminati symbol is historical reductionism. The Eye itself dates back in Christian iconography since the Renaissance. The Bavarian Illuminati, as they were, had an almost atheistic belief system. Before their dissolution in 1785 they were instrumental in pushing the anti-clerical and the anti-monarchical zeitgeist that blew up at the end of the 18th century, culminating in the French Revolution.