HCG is an acronym that stands for Human Chorionic Gonadotropin. It's a hormone produced by fetuses in women and also in the pituitary gland of both males and females. It is similar to LH (lutenizing hormone) so it is commonly used to induce ovulation in females and production of testosterone in males.
From Wikipedia:
In the world of performance-enhancing drugs, hCG is increasingly used in combination with various anabolic androgenic steroid (AAS) cycles. As a result, hCG is included in some sports' illegal drug lists.
When exogenous AAS are put into the male body, natural negative-feedback loops cause the body to shut down its own production of testosterone via shutdown of the hypothalamic-pituitary-gonadal axis (HPGA). This causes testicular atrophy, among other things. hCG is commonly used during and after steroid cycles to maintain and restore testicular size as well as normal testosterone production.[25]
High levels of AASs, that mimic the body's natural testosterone, trigger the hypothalamus to shut down its production of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Without GnRH, the pituitary gland stops releasing luteinizing hormone (LH). LH normally travels from the pituitary via the blood stream to the testes, where it triggers the production and release of testosterone. Without LH, the testes shut down their production of testosterone.[26] In males, hCG helps restore and maintain testosterone production in the testes by mimicking LH and triggering the production and release of testosterone.
If hCG is used for too long and in too high a dose, the resulting rise in natural testosterone will eventually inhibit its own production via negative feedback on the hypothalamus and pituitary gland.
References:
25^ Williams, Lance (May 8, 2009). "Manny Ramirez suspended; testosterone ratio exceeded limits; fertility drug reported". San Francisco Chronicle.
26^ van Breda E, Keizer HA, Kuipers H, Wolffenbuttel BH (April 2003). "Androgenic anabolic steroid use and severe hypothalamic-pituitary dysfunction: a case study". Int J Sports Med 24 (3): 195–6. doi:10.1055/s-2003-39089. PMID 12740738.