The presence of a small, fluid-filled balloon in the scrotum is a typical sign of a condition called hydrocele, or hydrocele testis.

A hydrocele testis is an accumulation of clear fluid in the tunica vaginalis, the most internal of membranes containing a testicle (Pic. 1). A primary hydrocele (Pic. 2) causes a painless enlargement in the scrotum on the affected side and is thought to be due to the defective absorption of fluid secreted between the two layers of the tunica vaginalis (investing membrane). A secondary hydrocele is secondary to either inflammation or a neoplasm in the testis. A hydrocele usually occurs on one side, but can also affect both sides. A hydrocele is normally seen in infant boys, as an enlarged scrotum. 

A hydrocele feels like a small fluid-filled balloon inside the scrotum. It is smooth, and is mainly in front of the testis. Hydroceles vary greatly in size. Hydroceles are normally painless and harmless. Large hydroceles cause discomfort because of their size. As the fluid of a hydrocele is transparent, light shown through a hydrocelic region will be visible from the other side. Symptoms of a hydrocele can easily be distinguished from testicular cancer, as a hydrocele is soft and fluidy, where a testicular cancer feels hard and rough. Through diagnostic ultrasound the accumulation of fluids can be diagnosed correctly.

Sources

Hydrocele testis ―sourced from Wikipedia licensed under CC BY- SA 3.0
Hydrocele testis ―sourced from Fertilitypedia licensed under CC BY-SA 4.0
Hydrocele testis ―sourced from Wikidoc licensed under CC BY-SA 3.0
The testicle, Gray´s anatomy ―by Henry Vandyke Carter licensed under CC0 1.0
Hydrocele ―by Internet Archive Book Images licensed under CC0 1.0
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