What's it mean when "s" is added at the end of a year like here?

Native speaker’s answer
Rebecca
If "s" is added onto a year that ends in a ten, they're referring to the whole decade. So here, the 1840s means 1840 to 1849. Ex: In the 70s, tie-dye shirts were quite popular. => 1970-1979 Ex: The 1920s is known for being the "Jazz Age." => 1920-1929