'Benign' means 'something doesn't cause any harm'. You can only really say you have benign MS after you've gone 15 years or so without many symptoms and you've got little or no disability.
A diagnosis of benign MS doesn't guarantee you'll be free of problems. You might still get things like fatigue or problems with your memory or thinking.
MRI scans can still show MS is damaging your brain and spinal cord even if yours seems mild. After many years of your MS appearing to be 'benign' it's possible you might have a relapse.