Chat links are special strings that the chat frame translates into clickable buttons in the string, which you know as links or more typically hyperlinks (like on websites).
Without the chat window processing them, they look something like this:
Code:
|Hitem:7073:0:0:0:0:0:0:0:80:0|h[Broken Fang]|h
|H denotes the start of such a link, and what follows is the metadata about the link (such as the type of link, like "item", the item ID in this case, and the item's attributes). |h shows up twice, once to denote the end of the metadata, which is then followed with what should be displayed in the chat window (in this case [Broken Fang]), and ends with a final |h.
|H and |h here are escape sequences, similar to how |c is used to start a color sequence and ends with |r.
More information about links, and in particular itemLinks and itemStrings:
https://wow.gamepedia.com/UI_escape_...ces#Hyperlinks
https://wow.gamepedia.com/ItemLink
https://wow.gamepedia.com/ItemString
This is similar to how links are made on websites, in which they are made up of anchor tags. You'd see "click here" and you'd be able to click it, while the code to do that would be this:
Code:
<a href="www.wowinterface.com">click here</a>