When you think of the Chinese character 口（kǒu）you’ll no doubt think of “mouth”, though 口 is also used as a general word to represent openings. The image below shows three examples of how 口 can be used to represent an opening, these being types of entrance or exit:
Image of a weigh bridge 地磅（dì bàng） office
入 means to go into, or to enter, so naturally an opening that you go into is a “entrance”. 入口 could be used for a door, an entrance for vehicles into a car park, or anything really – it’s the catch all “entrance” word.
The opposite of 入 is 出, which means to come, or go, out.
Worth noting here is that even though 出口 also means to “export”, 入口 does not mean to import. Import in Chinese is 進口（进口 jìn kǒu）, 進（进） being another way to say “enter” or “go into”.
Another common word for entrance is 門口（门口 mén kǒu）, though this is specifically a doorway – The Chinese character 門（门 mén） means “door”.
Lastly, and also seen in the above image, is 窗口（chuāng kǒu）, which is a window. While a standard window is called a 窗戶（窗户 chuāng hù） in Chinese, a 窗口 is more like the window at a post office or bank, or the drive-thru at a McDonalds, a type of Window that you might interact with people on the other side.
Also, interestingly, a 窗口 can also be a contact person for a business or organisation, for example – 他是那家公司的窗口（tā shì nà jiā gōngsī de chuāngkǒu）”He’s the contact person for that company”.
In the ancient oracle character, the “口” was a mouth opened waiting something to eat. So the “口” become the counting unit when the government want to know how many people(有多少人口) in their country. They have to know that every man in their country means a mouth to be feed.
There is a saying talking about this: 民以食為天.
The idiom at the end 民以食為天（民以食为天 mín yǐ shí wéi tiān）means “the people view food as everything”, or food is the most important thing for the people. It uses a great sentence structure that is well worth learning:
Using the idiom above as an example (以這個成語為例）: 民 is short for 人民（rén mín）which is means “the people”, 食 is obviously 食物（shí wù）, or “food”. 天 means sky, but can be anything from “god” and “heaven” to “nature” and “destiny”, so “everthing” or the most important thing is a good translation here. Now if we add in the 以…為… structure we get:
The people 以 food 為 everthing -> The people view/use/regard food as everthing.
I think another post looking more at the uses of 口 is in order, keep an eye out in the coming weeks for this!