Early Meiji Restoration is one of two mutually exclusive decisions 
Japan can take, the other being The Meiji Restoration.
Early Meiji Restoration requires Japan to be at peace and not being in a sphere of Influence, while the proper Meiji Restoration requires Japan to be in a sphere of influence.
This means that one can enact the Early Meiji Restoration on the very first day of the game, while one have to wait for The Meiji restoration. (The Meiji Restoration can however be taken immediately at the 1861 starting date since Japan is in 
USAs sphere of influence)
Both Early Meiji Restoration and Meiji Restoration gives Japan enough research points for a reform, 10 prestige and a modifier that gives Japan 30% research points until the end of the game. On top of that Early Meiji Restoration also gives the economic reform 'Land Reform' at the cost of 0.5 Militancy for liberal POPs, 7 militancy for Conservative POPs and 9 Militancy for Reactionary POPs.
While the early Meiji Restoration technically awards Japan with more, 7 and 9 militancy respectively is a lot and one risks a reactionary riot, that will set the Westernization process back a lot. The player has to value the risk against the extra gain.
Early Meiji Restoration and Meiji Restoration is overall two very good decisions, that arguably makes Japan the easiest uncivilized country to play as.
The Ingame describtions of Early Meiji Restoration and Meiji Restoration state the same, which is:
Japan is traditionally considered to have been founded by Emperor Jimmu in 660 B.C. and while the Imperial Lineage was still considered unbroken since then the Imperial Office had throughout the Edo period been reduced to a revered and respected but ultimately politically powerless institution. By embracing a restoration of Imperial Rule along with a programme of swift industrial modernization constitutionalism and economic freedom Japan might be able to break the fetters placed upon her by the impositions of the Western World.