What is rough chop used for?
Making a soup with potatoes is a great example where a rough chop may be employed. The potatoes are an integral part of the dish; as such, you want to ensure all of them are cooked to the same consistency (neither overcooked or undercooked).
What does it mean to rough chop an onion?
It is a bit of a loose term, but if you where cutting an onion then the size would be approximately 2cm, for herbs the size just under 1cm. Roughly chopping usually has more leeway on the exact size, and variations wont affect the taste or texture of the dish much.