Why Read/write rules and round tripping?

Hello,

I'm planning to set up a documentation project using DITA and I'm evaluating which editor would be better. I don't know Framemaker and one of the things I don't understand is what are the read/write rules? When do I need them? What does it take to define them?  My output will be PDF files and the content will be translated in more than 20 languages.

Many thanks if one of you can clarify that point for me,

Regards,

Annette