What is the meaning of plain?

Answer 1

As an adjective, ordinary.

As a noun relating to land, a plain is an area of land not significantly higher than adjacent areas and with relatively minor differences in elevation, such as a flood plain along a river or the Great Plains of the Mid-western United States.

Answer 2

"Plain People" refers to Amish, Mennonites, Quakers and other groups of Christians who hold to a simple life-style and do not artificially beautify their clothes or possessions.