A huge post for a small small stamp,
I found the info on a forum for stamp collectors
The origins of this stamp are in the June 21, 1948 Currency reform by the western zones of Germany, which caught the Soviet with their pants down and lead to a rushed currency reform in the Soviet (and Berlin) on June 24th (June 25th in West Berlin). Emergency stamps were over- printed in many post offices (the so-called "District Handoverprints") every night to meet the next day's demand, until machine-overprinted stamps became available on July 3rd.
This immediately lead to the Soviet Blockade of West Berlin and the Berlin Airlift (Operation Vittles). The population of West Berlin suffered greatly during the time of the blockade, which was not lifted until May 12, 1949. (The Airlift continued until Sept. 30.)
As a means to defray the costs of this massive resupply operation and to provide continuing assistance to the people of Berlin, the German parliament (with military government approval) passed a law requiring a 2 Pfennig tax on various classes of mail. (Covers franked contrary to this law exist, e.g. Notopers used to pay postage and ordinary stamps used to pay the tax.)
The tax was to be paid ONLY by the "blue flea" stamp, first issued on Dec. 1, 1948 and inscribed "NOTOPFER / 2 BERLIN / STEUERMARKE". This translates to "Emergency Victims / 2 Berlin / Tax Stamp". Although this is technically a tax stamp, it was in fact sold by the post office and it had no use other than on mail.
Covers franked contrary to this law exist, e.g. Notopers used to pay postage and ordinary stamps used to pay the tax.
The Notopfer was not required on all pieces of mail. Initially, the Notopfer was required only in the Bizone (combined American and British zones). It was later used in the French Zone. It was never used in Berlin itself. It was not required on mail to Berlin, on mail to the Soviet Zone and on mail to foreign destinations. Examples used inadvertently to the Soviet Zone or Berlin were often defaced and returned by the Soviet Zone authorities (and are very collectable).
For mail that was not exempt per the above, initially every class of mail required the Notopfer. This represented a 10% tax for letter mail, but was a whopping 50% for printed matter. Businesses complained loudly about this and printed matter was later made exempt.
Even official mail that was otherwise free required a Notopfer!
The period of use for the Notopfer was Dec. 1, 1948 to March 31, 1956, thus surviving the transition from occupied Germany to Federal Republic. Overall, more than 10 billion of the "fleas" were printed.