A clickbait is when someone makes a thread and then, for the thread's description, puts in a topic that has nothing to do with the title, "clickb8", or "clickbait".
This is usually done to get other people's attention.
A good way for detecting clickbait's is from this thread by DoctorExpo.
How to detect a clickbait
"Just look at the number of views than replies. If there is more views than replies, then it's most likely a clickbait."
Some examples of clickbaits are:
Reason: It's pretty much obvious.
Reason: The starter of that thread is obviously a human, not a dog.
Reason: McDonald's don't make their burgers that way.