This can refer to the range of mountains that stretch from Laramie Mountains to the south of Casper Wyoming to just south of Pikes Peak near Colorado Springs. The mountains are quite impressive, rising from the plains of Colorado. Along their 250-mile length they extend through many wild areas and Rocky Mountain National Park. (One of the crown jewels of Colorado and the US National Park System.) They are part of the larger grouping of mountains known as the Rocky Mountains.
It can also refer generally to the area bounded by the mountains and north of the Palmer Divide. It the location of Denver and its far-flung suburbs which seem intent on covering the entire area all the way to Cheyenne Wyoming.
Boulder is already considered part of the metro area, as is the small down of Parker in the south. Up next for absorption into the Metro area are Longmont and Castle Rock.