The seminal vesicle is a slimy and waterly stuff that goes trough your penis it's function is to secrete a significant proportion of the fluid that ultimately becomes semen.
In male animals, the seminal vesicles are two pouches, each connected to one vas deferens. They produce an additional fluid that combines with the sperm to produce semen.
In human males and in many other sexual animals (even earthworms) the seminal vesicle provides the fluid that becomes male semen. In some they also store the semen for later release.