Carl, the Gleason score is the grading system that your cancer team uses to determine the aggressiveness of prostate cancer. Knowing the Gleason score can be used, along with other information gathered during diagnosis, to help recommend the best treatment options.

Here's further explanation from the Prostate Conditions Education Council (PCEC)
https://www.prostateconditions.org/about-prostate-conditions/prostate-cancer/newly-diagnosed/gleason-score

"Typical Gleason Scores range from 6-10. The higher the Gleason Score, the more likely that the cancer will grow and spread quickly.
- 6 or less describe cancer cells that look similar to normal cells and suggest that the cancer is likely to grow slowly.
- 7 suggests and intermediate risk for aggressive cancer.
- 8 or higher describe cancers that are likely to spread more rapidly, these cancers are often referred to as poorly differentiated or high grade."