Link bait refers to website content that is created in order to increase the number of “inbound links” to your website. (This is a key part of any link building campaign.) In other words, link bait is content that is optimized to be incredibly useful, funny, insightful, surprising, awe-inspiring, or uplifting. The reason people make link bait is because Google considers inbound links to be a major factor in its determination of your website’s overall rank compared to your competitors’ websites. You should think of inbound links as a type of important validation that Google likes to see. Essentially, Google is scared of spammy websites, and in order to avoid including spammy websites, they want to see if other people are passing judgment in the form of links to any website on the Internet.
There are a number of different tools that search engine optimization professionals use to count and identify inbound links (both for websites they own, and for websites that they want to outrank). For instance, I can see that the New York Times (which has incredibly high ranking from Google) has exactly 91,978,629 inbound links from 98,518 different domains.