Being in a place where things are done differently than you are used to can cause confusion, anxiety and a feeling of uncertainty, even if the environment you are in is actually better than the one you feel more comfortable in. All those feelings can be described as a culture shock as a person is cut off from the cultural patterns that are familiar and has to adapt to something new.
Most often the term is used when talking about moving to another country or traveling. There are ways to soften the culture shock and one of them is asking people to share their own experiences on Reddit so you can prepare for it. We’re not sure what was the motivation of Kampfhamster248 to ask “What was something that shocked you when you visited a foreign country?” but it sure does help to know what people in the thread had to say before going somewhere for the first time.