The posterior peripheral zone is the area round the urethra, and is where most prostate cancers originate (about 75%) - low signal means there is an abnormality ie the signal is not bouncing back so much because the cells are not reflecting as healthy cells would. T2 and diffusion are different ways of reading the scanned images - diffusion is looking at how light is reflected off water - healthy cells have more water than cancerous cells so restricted diffusion is a concern. PIRAD 5 means there is a suspicious area of at least 1.5cm and/or signs that it may have escaped.
That is the dry interpretation. However, T2 scans with low signal cannot differentiate between prostate cancer, BPH and prostatitis. Low diffusion (less water in the cells) could be cancer or prostatitis. So all these results are telling your specialist is where to concentrate the needles when he does the biopsy - you now know that there is a suspicious area in the most common part of the gland, it is a decent size for the urologist to aim at so he should get good reliable samples - but, as likely as it is now looking, only a biopsy is going to tell you for sure that it is cancer.