This basically means that if you do a good job, the reader will go anywhere with you. If establish good parameters and establish the world well, the reader will travel with you. The reader willingly suspends their belief to travel through fiction (and in particular, fantasy and science fiction) to go on an adventure.
And readers know when you start to lie and deviate from the rules.
Basically, we have two rules with which to work.
- Make the world believable with lots of details.
- Make the people believable with lots of humanity.
The interesting thing about the willingful suspension of disbelief is when a writer breaks them.
Take Brazil again. You can almost point to the moment in the movie when you can't believe in the world anymore--it has broken from the rules. We know it, the filmmaker knows it, and we start to roll our eyes. We know this isn't going to work.
And then, we know the filmmaker is just playing with us. Terry pushed us and played with us, and we realize it.
And even though Terry broke the rules, we play along. Mostly, we're okay with it.
Here's a video that should be funny but isn't really. Nonetheless, it has a great breakdown of the guidelines of suspension of disbelief: