Could you explain what does a gleason score of 7 mean?

Gleason score: Gleason score is a determination of aggressiveness if prostate cancer as determined by a pathologist interpretation of tissue from a prostate biopsy. He determines the least aggressive glands and gives a score of 1-5, then he looks for more aggressive glands or cells and gives a score of 1-5. The combined score is the gleason score from 2-10. Score of 2-4 is well diff, 5-7 mod diff, 8-10 poor d.