Hello Treina!
Welcome to the world of revenue stamps. Revenue stamps, such as the one you posted, was first issued by the government to help pay for the civil war. A tax was imposed on a variety of things. The stamp was placed on the item purchased and cancelled as proof of payment of the tax.
Wikipedia can give you a good grounding in the history of the topic so that you can learn the basics:
https://en.wikipedia.org/wiki/Reven...nited_StatesThe stamp you posted has the Scott catalog number of R6c. It is one of the more common first issue revenue stamps. This first group of stamps (first issue) was used primarily from late 1862 until the early 1870s. Although this stamp was issued to pay the 2c tax on writing a check (yes they paid a tax to write a check!), it was often used to pay for a variety of transactions that also costs 2c, such as a photograph (search
ebay for CDV and stamp for an example).
It is easy to get addicted to revenue stamps. You can't just stop with one.