Presumably this woman, who is quite a hefty girl and obviously capable of looking after herself, is breaking asunder the chains that had held her. The design was first used in 1920, shortly after Czechoslovakia had been formed out of the wreckage of the Austro-Hungarian empire. So the design probably seemed very relevant to the new nation.
A lot of stamps feature personifications of the country. For example, British stamps often feature Britannia ruling the waves. Early German stamps featured Germania, a woman with some sort of armoured helmet and steel breast plate. New Zealand stamps had a figure known as Zealandia - a European woman often depicted with the classical staff carried by Mercury. Oh, and that's another interesting thing. Some of these figures have names and others don't.
If anyone comes across any other allegorical figures of nations, please let me know.
See also - Is this the ugliest portrait of Queen Elizabeth?