What does a coin MS-64 DMPL mean?

MS= Mint State

DPML= Deep Mirror Proof Like

70 is perfect. 68 is 1 in 100000 and rare. The higher the number, the more value the coin has among others of the same year and mint. Older coins are tougher to find in higher grades because they were not made for collectors like some coins have been in the last 50 years.