Read mode in MIUI ROM is originally design for people who are likely to read or watch videos before going to sleep.
The device in normal brightness emits bluer wave which can lead to the deterioration of sleep quality. As well as it will take a longer time to fall asleep.
In MIUI read mode, the device emits lower blue light and lessen the normal brightness to prevent any kind of damages to your eye and brain.
Here is an image showing the difference between normal mode and read mode:
How to enable MIUI reading mode:
Simply turn on settings and notifications shutter and tap on the icon with the eye (Read Mode). Here is an image showing how to enable MIUI reading mode;
Configure reading mode:
You can also configure the reading mode according to you fit. go to the Settings menu mode (long tap, hold on the icon). Then configure an adjustable mode to your own. Here is an image showing how to configure reading mode: