Centering means the center of each line will be in the center of the page. So a shorter line will start closer to the center and a longer line will start farther.
This is a roughly centered line.
You don't want to do this to regular paragraphs since, it means the left edge will wander all over creation.
Centering is used only for titles or captions, usually.
The suggested text indent will have the effect of starting each paragraph like this:
This is a paragraph test test
This is the next line of the paragraph.
Some like the effect, some do not. It is up to you, since it makes not too much difference in usability, depending on how long the paragraphs are.
Check out
www.w3schools.com. They have html and css tutorials and their try it section allows you to see the effect of changes. You need to take a look at it, since Sigil assumes you have some knowledge of it.