Redmine is an open-source, web-based project management tool used for tracking project progress, managing tasks, tracking issues, and collaborating with team members. It offers features such as Gantt charts, calendars, time tracking, and integration with other tools like Git and Subversion. It is highly customizable and can be tailored to meet specific project needs. Redmine is commonly used by software development teams, but can be used in any industry that requires project management. As a specialized skill, proficiency in Redmine would involve understanding its features, management of projects on it, as well as integration with its connected tools.