Haha - I was just having this conversation about JR flavors yesterday. There are often people who don't know what Jolly Ranchers are or care about them, so here's my attempt at explaining:
Jolly Ranchers are a hard candy in the following flavors: Cherry, Apple, Blue Raspberry, Watermelon, and Grape. There are pockets of people who have Jolly Ranchers regularly, and those who know them who get Jolly Ranchers regularly - for example, if your ears have trouble readjusting after a plane flight, sucking on a Jolly Rancher may help.
Now, among those pockets of people, there are strong allegiances to the "best" flavors. Some people really like the Blue Raspberry. Some people like everything but one or two flavors. But the One True Jolly Rancher Flavor Aficianado understands that the best flavor is, of course, Watermelon.
Unfortunately the casually racist and actually super racist people of the world love to associate Watermelon with black people as a racist trope. (See trump MSG rally "comedian"'s comment to random black trump supporter in the crowd)
So, in this comic, the white guy shares a Jolly Rancher with the black guy and it's not the Watermelon flavor - specifically to get around the baggage of the trope.
Whether it's funny or not is of course subjective, but it's funnier if you have accepted into your heart the truth that Watermelon JR is the best flavor.