iPad side switch screen rotating lock.
One of the things that I find frustrating when reading on my iPad is the screen orientation reacting to the orientation of the iPad. When you rotate your iPad the screen tries to change to keep the screen content vertical. It can be frustrating if you want to turn your iPad but keep the content locked in one position.
To stop your iPad screen rotating when you move your iPad about you need to enable ‘portrait orientation lock‘. There are two ways to enable portrait orientation lock on an iPad:
The first way is the software way: see this post
The second way is the hardware way.
You can use the switch on the side of your iPad to enable and disable portrait orientation lock. This is very handy as sometimes you need it on (eg if you are lying on a sofa, or in bed) and sometimes you want it off – for example if you are reading a website or watching a video in portrait mode and flip your iPad on its side to view the video in a larger size.
Now you can read while lying down on the sofa, or in bed without your screen flipping about!
Sources: Apple Support