The primary male sexual organs. They are responsible for the production of sperm, which are transported via semen to the uterus of a female, at which point they are used to impregnate her. The testicles are also a major male erogenous zone.
Technically, when testicles are tagged, it is actually the scrotum surrounding the testicles which is visible in most cases. A true view of the testicles would require a cross-section.
source: danbooru wiki