"Hack" in this context means a shortcut, a cheat, an unconventional way of doing things that might get the result but is not the proper way.
It is a relatively new word used this way and is drawn from the context of computer hacking. A "hack" (noun) in computing terms usually involves introducing some new code to an existing system to make it do something it was not meant to do, or to achieve a result faster.
In this new context, the term can refer to any non-computing aspect of life where something different from the norm is done to get a different result or save time. This wider use of the word has spawned the compound word "lifehack", which is defined as "a strategy or technique adopted in order to manage one's time and daily activities in a more efficient way".
In your example, it is talking about a suggestion to "skip lines" to make reading faster, which will certainly speed up the process of reading, but as a "hack" it is not reading the way you are meant to, and like unstable computer code, it may produce an unexpected result.