What's that mean to put "s" after the year like 1800s in this sentence?

Native speaker’s answer
Rebecca
Ah! They're indicating all the years between 1800-1899, which means that it's plural "years," so the "s" is added. Ex: I wonder what it would be like to travel back to the 1200s. Ex: In the 1900s, a lot of technological advancements were made.