Migration is the process of moving information stored on a Web content management system (CMS), Digital asset management (DAM), Document management system (DMS), or flat HTML based system to a new system. Flat HTML content can entail HTML files, Active Server Pages (ASP), JavaServer Pages (JSP), PHP, or content stored in some type of HTML/JavaScript-based system and can be either static or dynamic content.

As a term in WordPress migration in plain English means the move of a site from one server to another. There are many useful plugins out there in order to help you with this task. Most of them are completely free on the WordPress plugin repository