It means you have reached the top level of the Marriott Rewards program, congrats!
In theory there's some additional chances of upgrades, many premier members (including myself) have had good upgrades after attaining this level. There's also a new phone number to call in, which is rarely if ever not directly answered in my experience. Otherwise there's not a ton of differences from Platinum but if you click on the benefits page on Marriott.com it used to tell you all the perks but now its just a plain screen for me with the PP phone number.
They treat you pretty well at Platinum level. Extra points. room upgrades, often staff will give away a bottle of water if you stopped by the front desk to ask a question in the morning. I even had a change in plans, forgot to cancel a room before the deadline and when I called was told, "No worries. You are a Platinum member."