A separate mobile URL (often called an “m-dot” site, such as m.example.com) is a distinct, separate website version specifically built for mobile visitors, as opposed to a single responsive design serving all devices from the same URL.
This older approach requires significant extra technical maintenance, including proper redirects between desktop and mobile versions and correct annotation tags, to avoid duplicate content and indexing confusion between the two separate sites.
Given the added complexity and maintenance burden, most modern websites have moved away from separate mobile URLs in favor of responsive design, which Google also generally recommends for simplicity.
We typically recommend migrating away from separate mobile URLs within our Technical SEO service.