"Soft roe" is a euphemism for fish sperm, in cases where it is eaten by human beings, by analogy with roe, which is the term for fish eggs when eaten by human beings.
Since fish sperm is, as you might imagine, rather tiny and therefore somewhat hard to keep track of, soft roe is most commonly consumed together with seminal fluid, often while still contained within the fish's spermary organ.
Soft roe thus generally appears as a salty, milky white fluid.
Fish whose sperm is most commonly consumed by humans include cod, herring, anglerfish, and tuna. Soft roe is noted as a delicacy in British, Russian, and Japanese cuisines, among others. "John West" is a noted brand name for soft roe in Great Britain. Soft roe is known as shirako (literally, "white children") in Japanese. In Sicilian cuisine, tuna sperm is known as la lattume and is used as a topping on pasta.