I was assuming you are under your GP, but if you're already having Active Surveillance, then the followup would be by Urology.
A DRE (Digital Rectal Exam) is where a doctor's finger (digit) is inserted into your rectum to feel your prostate.
Why did you have the MRI scan? A standard MRI scan doesn't show prostate cancer very clearly, but if the scan was specifically to check for prostate cancer, a multi-parametric MRI scan would have been done which shows up suspicious areas of the prostate. MRI can't diagnose prostate cancer, just identify suspicious areas which might need more investigation.