What does 'the devil's advocate' mean?

[Watie]

English Language Tests, Intermediate level

ESL/EFL Test #283 "English Slang Idioms (5)", question 9

Jennifer decided to play the devil's ......... in order to add another perspective to the one-sided conversation.

(a) buddy
(b) helper
(c) favorite
(d) advocate

English Language Tests, Intermediate level

ESL/EFL Test #283 "English Slang Idioms (5)", answer 9

Jennifer decided to play the devil's advocate in order to add another perspective to the one-sided conversation.

Correct answer: (d) advocate

Your answer was: incorrect
Jennifer decided to play the devil's buddy in order to add another perspective to the one-sided conversation.
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What does 'the devil's advocate' mean?
Thanks

[Beeesneees]

Someone who takes the opposite side just for the sake of argument or discussion.

Jennifer might not have agreed with/believed in what she was saying, but she was stating the opposite view to whoever she was talking to in order to make sure the topic was fully discussed.
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"Inside every old person is a young person wondering what happened."
Terry Pratchett