You use rather when you are correcting something that you have just said,
especially when you are describing a particular situation after saying what it is not.
He explained what the Crux is, or rather, what it was.
разные степени хорошести:
quite good - хорош на 60%
rather good - хорош на 80%
very good - хорош на 100%
You use rather to indicate that something is true to a fairly great extent.
I grew up in rather unusual circumstances...
The first speaker began to talk, very fast and rather loudly...
I'm afraid it's rather a long story...