The phrase "open plains" refers to large, flat areas of land without many trees or hills. It means that the land is wide and vast, with clear views over long distances.
It is surrounded by a beautiful landscape of mountains, hills, wide open plains, lakes, rivers and forests. (welovedates.com)
Next, close out of your word - processing program and open the plain text file you've just created. (myperfectresume.com)
The film never really captures the conflicting environments of the claustrophobic old Weston homestead and the free wide open plains of Oklahoma. (moviereviewsfromthedark.com)