What does 的 mean in 你们在哪里认识的? 你们在哪里认识的?
Nǐmen zài nǎlǐ rènshi de?
Where did you meet?

What does the character 的 mean in this sentence?
One of the functions of 的 is being an adjectival marker that marks a phrase as an adjectival phrase
我们在中国认识  = We met in China
的 marks 在中国认识 as an adjectival phrase that describes 我們
[我們 (topic)] + [在中国认识的 (comment)] = [We] (are someones) [met in China]
你们在哪里认识的? just replaced 中国 with 哪里. 在哪里认识的 is still an adjectival phrase in an Interrogative sentence
Q: 你们(是)在哪里认识的?
A: 我们(是)在中国认识的