According to the inquiry, I know this means that this is a joke. "Stem", a network term, means punchline. For example, "rotten stalks" means that this is not funny, and "spreading stalks" is to pave the way for later jokes. Secondly, take "stem" as "bridge node". Some common scene fragments have certain paragraphs or slots.

"gěng" is a typo of "gén" in network language, which means "laughing point, foreshadowing, or something with special meaning or irony". It often exists in variety shows and networks, and it also appears from time to time in daily life.