A varicocele occurs when there is dilation (stretching or widening) of the veins on the spermatic cord. The spermatic cord is a bundle of structures which help deliver blood to and from the testicle, as well as carry semen produced by the testicle.
The stretching of these veins can occur when the internal valves do not maintain a sufficient rate of blood leaving the testis. This can lead to swelling in the scrotum, discomfort, and even infertility.
These seem to occur mostly in men between ages 15-25 but can occur in men of any age. Treatments include conservative management, angiographic management (going through a vein to obliterate the culprit veins), or surgery to address the issue.