Normally a word belongs to a certain part of speech. But some words have the grammatical function of two of more parts of speech. Here are two examples: For example: the word “恋爱” is used as a noun in “谈恋爱”, while it is used as a verb in “他们恋爱了”. The word “给” is used as a verb in the sentence “给他一只笔“ while “给” is a preposition in the sentence “给我们介绍”. In the phrase 内容丰富,丰富 is an adjective, meaning “rich”, while in the phrase 丰富内容, 丰富 becomes a verb meaning “to enrich”. In this case, 丰富 belongs to two different parts of speech; adjective and verb. This is called conversion of parts of speech, i. e. they belong to different parts of speech without any change in form.
If the meaning of a word in different sentences remains unchanged, it is not considered conversion although it has different functions. For example:我们学习 and 学习很重要, in both sentences,学习 is a verb.