Webster 1913 seems tight-lipped on the matter (as was
custom in those times), so I guess it's up to me to explain these
delicate things.
Cowper's glands are also known as bulbourethral glands. There are two small (1 cm in diameter), internal, pea-shaped glands beneath the prostate, near the base of the penis. They are made of a cluster of small tubes, supported by muscle fibers and elastic tissue. They each have ducts which empty into the urethra.
Any of you who have first-hand experience (no pun intended) with the basic operation of the penis with respect to the physical aspects of human sexuality have probably seen the secretion of the Cowper's glands. It's a thick, clear lubricant which is secreted when the male is aroused but before orgasm. It is sometimes eloquently described as pre-cum. In terms of quantity, enough is secreted to be messy, but the amount of fluid is small compared to the amount of semen at ejaculation.
The secretion is thought to clean the urethra of any remaining urine before ejaculation, and it may also contribute to making semen less watery. Generally, the overall purpose of the Cowper's glands seems to be protection of sperm, so that the tiny proto-humans may more likely survive their dangerous journey "upstream".