mid-century modern

Someone used the term "mid-century modern" to describe some furniture at an estate sale.

It is the year 2019 on the Gregorian calendar. I am living in the first half of the 21st century. This word makes no sense. I go look this up.

From the WikiPedia entry:

... the design movement in interior, product, graphic design, architecture, and urban development from roughly 1945 to 1975."

What!!!?? I still don't understand.