内容(nèi róng) - content, substance in Mandarin

内容 (nèi róng) is the substance of something as opposed to its outward shape: what a book, a speech, a film or a web page actually says. It pairs naturally with 形式 (xíng shì, form), and Chinese uses that pair far more readily than English uses "content and form".

In practice you will hear it most often with 的 in front of it, as in 会议的内容 (the substance of the meeting) or 信的内容 (what the letter said). It is a mass noun: you do not count 内容, you describe it as 丰富 (rich), 简单 (simple) or 差不多 (much the same).

The trap for an English speaker is the modern web sense. English "content" has drifted into a job description, as in content creator or a content strategy. Chinese 内容 has picked some of that up too, but on its own it still means the substance of one particular thing, so a sentence like 我做内容 lands as vague where an English speaker expects it to be self-explanatory.