Kaepernick's gesture—kneeling, rather than standing at attention, during the national anthem—communicates in symbol, but it can also be understood literally.
The anthem's third and fourth verses, rarely sung, are widely held to indicate Francis Scott Key's delight in the prospect that winning the War of 1812 would keep the institution of American slavery alive. As Jon Schwarz writes at The Intercept: "The reality is that there were human beings fighting for freedom with incredible bravery during the War of 1812. However, 'The Star-Spangled Banner' glorifies America’s 'triumph' over them—and then turns that reality completely upside down, transforming their killers into the courageous freedom fighters."
It's an argument so convincing that even Dilbert creator and alleged idiot Scott Adams agreed (kind of).