Here's how I figured out what it meant.
In a WordPress website, WordPress has these little snippets of code to generate the HTML.

For example:
[Generate Header]
[Generate Body]
[Generate Comment Box]

Let's say you wanted to put a social media banner between the Header and the Body. You would write the code, and then 'hook' into the body. So now, the stack is like this.

[Generate Header]
--> Your custom banner code
[Generate Body]
[Generate Comment Box]