Clickbait is a headline that almost forces you to click on it -- it promises a quick substantial reward, a juicy, very focused tidbit, for just a click...just a little, simple click... come on, you know you want to...
Like offering a wiggling worm (impaled on a hook) to various fish swimming around in a body of water...

LOSE TEN POUNDS OVERNIGHT BY DOING THIS
TRUMP'S BIGGEST MISTAKE AT LAST NIGHT'S EVENT
or...
WHY TRUMP WANTS TO MEET ALONE WITH PUTIN (taken from CNN.com this morning!)

Headlines have always been clever, attention grabbing, etc. -- but lately, actual clickbait type headlines have become popular even on "professional" sites like CNN, not to mention many crap news sites, which could legitimately be called fake news (the real fake news, not the Alternative Media which the Left likes to disparage as fake news)
I often see such headlines on the side/bottom as I'm trying to read a REAL article someone sent me to.

Clickbait forces you to click on the link to get the simple answer: "what is the article about?"

Headlines SHOULD, and USED TO, offer you a short summary of the article, so you could read the article or not, depending on if you had time. But today advertising is based on pageviews, clicks, etc. so the modern websites have resorted to denying us the courtesy of being able to scan headlines to see what we're interested in. If we scan the headlines today, we won't know much unless we click and/or read each of the stories!

Much worse than that - headlines also tell you how to think these days. You have the obvious:
"America is becoming less Christian, and here's why that's a good thing!" Very clearly telling the reader how to think on an issue
But you also have a more common and more subtle type like this:
"Trump criticised for comments on yada yada". A fair and balanced media would just tell you what Trump said and let the reader decide for themselves whether it was worthy of criticism or not, but by saying that Trump was criticised for it, they're implying that it's something worthy of criticism and subtly trying to get the reader to agree.