Learn French: Voici le vase dans lequel elle met les fleurs. - Here is the vase in which she puts the flowers.

lequel means "which" and it has to agree with the noun it refers back to. Here, it refers to le vase (the vase), which is masculine and singular. If we were talking about a feminine noun like la boîte (the box), we would say dans laquelle.

Why do we need lequel at all? Couldn't we just say dans que for "in which"?

In French, the basic relative pronoun que (that/which) cannot be used after a preposition like dans, sur, or avec when talking about things. After a preposition, you must switch to a form of lequel.

Could I use où instead of dans lequel in this sentence?

Yes! Because le vase is a physical container, you could also say Voici le vase où elle met les fleurs (Here is the vase where she puts the flowers). Both are perfectly correct, but dans lequel emphasizes being specifically inside the vase.

I remember learning lequel as a question word meaning "which one". Is it different here?

Yes, it is playing a different role here. It can be a question word (like Lequel tu préfères ? - Which one do you prefer?), but in this sentence, it is a connecting word. When following a preposition in the middle of a sentence, it translates simply to "which".