A Type I region is a specific type of two-dimensional area in the plane where, for a given range of x-values, there exists a single upper and lower boundary defined by functions of x. This means that the region can be described as being 'vertical' in nature, allowing double integrals to be computed with respect to y first, followed by x. Understanding Type I regions is essential for evaluating double integrals over non-rectangular regions, as they simplify the integration process through clearly defined limits.