Lines
Explanation
This comic features an alien (a recurring SMBC character) marveling at a human ability. The alien explains that "the good news" is that humans can detect diagonal lines in any orientation, and that they can also detect horizontal and vertical lines, as well as curves.
A human then shows the alien what humans can create with this ability -- but when the paper is revealed, the alien sees "just a blank piece of paper" and is amazed ("AAAAAHHHH! AMAZING!"). The joke is that the alien is so impressed by the raw ability to perceive lines that even a blank page -- which to the alien may represent the potential for line detection -- is worthy of awe. Alternatively, the alien's visual system may be so different from ours that it cannot even perceive the drawing on the paper, seeing only blankness.
The comic satirizes the tendency to be overly impressed by basic cognitive abilities when described in neuroscience terms. Line detection (specifically orientation selectivity in the visual cortex) is a well-studied phenomenon in neuroscience, famously explored by Hubel and Wiesel. While it is genuinely remarkable at a cellular level, the comic suggests that marveling at the basic mechanism misses the bigger picture of what those mechanisms enable -- actual visual perception and art.