The MRI scan isn't definitive. It can only identify lesions (areas of abnormal tissue, not necessarily cancer), and possibly spread outside the prostate. So, you need a biopsy to be definitively diagnosed.
MRIs are scored using the PIRADS or Likert scales, 1-5. 5 does mean the radiographer thinks it likely you have cancer. MRI can't diagnose the aggressiveness, that's the job of the biopsy.