The reason most folks would know what hCG is because it's what pregnancy tests for. In males, however, there are slightly elevated levels of hCG after ejaculation, which is what ESPN's Adam Schefter reported. According to Will Carroll, that doesn't mean much.
HCG can be detected at 20 to 100 ppm. NFL threshhold is unknown, but is likely the standard of 50 ppm. Natural levels in men near zero.
hCG in itself does not make you bigger like what you think of with steroids. It's used after a steroid cycle to essentially "kick start" the testicles to start working again. It's not masking steroids or anything like that. It's a substance commonly used after a steroid cycle so, as Carroll suggests, Cushing was likely using during training camp if the test came in September.
It stays in your system for a short time so, as Carroll explains, it's "telling" that he was caught.
This is the same substance that got Manny Ramirez banned from MLB last season.