A doorway page is one of the most famous types of search spam. A doorway is a site (or separate pages on a site) whose main function is to redirect users to another site. The initial site is usually practically devoid of content and useless to the site visitor. These sites often contain a lot of keywords.
In some cases doorway pages turn up after a site is hacked. In this case a lot of similar pages are created without the site's owner necessarily even noticing.
Re-examine what the point of your site is and add some useful content or functionality that you plan to develop further in the future. Your site should answer user queries and present a variety of information and/or services, rather than just redirecting users to other resources.
If doorway pages turned up on your site because it was hacked, you will need to delete those pages. You can read more about how to protect your site from various attacks in the Help section.
After correcting any such violations on your site, click I've fixed everything in the Violations section in Yandex.Webmaster. This way the robot will know that you've made changes.
The I've fixed everything feature is available once per month for each individual site. Keep this in mind and only click it when you're absolutely sure that you've taken care of all the violations present on your site.
If several people have management rights to your site, a specific person should be delegated to re-send the site for checking at a particular time. None of the admins will be able to use the button again for a month.
Restrictions are usually lifted within a month after the violations have been addressed.
Click I've fixed everything. This will help the Yandex algorithms to correctly evaluate your site. If your site really isn't a doorway, then the restrictions will be lifted in due time.