Doorway Pages are defined as the low quality pages which are optimized to rank well for specific keywords that act as a door or bridge between the users and the content. Technically, these pages has the minimum value to the users and serve the whole purpose for boosting the SEO value.
The Doorway pages act as a gateway or a bridge between one section of your site and the other content focused on the landing pages.
Doorway pages are easy to identify as they have been primarily designed for search engines and not for users.
Creating doorway pages is a Gray Hat SEO Technique, which might be useful to boost the rankings in the search engines. It might help you, but it can also be risky.