I don't fully understand what the big difference is but one thing I can say for certain is that this only affects "Institutional Investor Funds".
That means if you are using a Money Market Fund at your bank (retail) chances are this has no effect on you.
This only affects Money Market Funds sold at brokerage houses, such as Vanguard, Fidelity, etc.
Some more that I know, but I don't know all of the why's:
*Money Market Funds have historically targeted a fund share cost of $1. So if you had $400 in the account, you owned 400 shares. Why the fund price is so relevant I'm not sure, but that's what this is all about. The Institutional Money Market Funds are now being told to allow that $1/ share to float, just like any other share in any other Mutual Fund. By definition there shouldn't be much float day to day in these funds, they are invested in CDs, Commercial Paper and other relatively stable assets.
*Some Money Market Funds are FDIC insured, some are not. The some of the ones that are will be at banks/ credit unions, but not all of them. Ones at brokerage houses will not be FDIC insured.