Chonk is an adjective that is used to describe something fat, with strong overtones of cute. Because internet, it is generally used to refer to cats, e.g., "that cat is a chonker". Doggos, birbs, trucks, airplanes, and round fluffy bois of any species may be considered chonking, but the platonic form of chonk -- in techspeak, a "heckin' chonker" -- is cat. For other entities to be chonk, they must be not just fat, but emphatically rotund.
Chonk appeared around about the summer of 2018 as part of the cute-speak fad broadly related to the doggolingo that emerged in 2014. It joins the preexisting ketteh speak that comes to us from the ancient days of lolcat, but shares more in tone with the doggos than the 'i can has' crowd.