MML, which originally stood for Mart's Markup Language but has now dropped the meaning of the first M, was created to make life easier for people maintaining sites or sections of sites which have a constant look throughout.
Many sites these days adopt a 'template'-based approach, with a constant set of navigation links, a logo and a heading etc. Creating a site like this is the bane of many a webmaster's life, making sure that all of the URLs are correct, and that the navigation system stays consistant.
Add to this the problems that occur when a style change is needed, whether this is the addition of a new section which must be added to the consistant navigation system, or complete restyling perhaps due to corporate rebranding, or just a desire to change things.
MML makes all this easier by separating the consistant navigation and style from the content. At its simplest, MML allows you to define a page template which all of your pages will use. For those who want to take it further, there are also possibilities for multiple different styles, variables which can be set at various different points and then inserted into the final document and access to server-provided information such as the referring URL.
All of this is done using markup very similar to HTML. In fact, most of the tags are actually HTML tags although they aren't parsed by a browser but rather the MML pre-processor, which is what builds an HTML document from all of the separate parts.
MML builds HTML pages based on templates and source files. The templates define a style which will be used for all of the site while the source files contain just the content.
What you put in the source files and what goes in the template is mostly up to you, but it is a good idea to put at least everything that doesn't change in the template. You may also wish to put some of the things that do change in the templates, since you can use variables which can be set for the entire site, and then overridden on a page-by-page basis where required. This could be handy when there is something that is almost always the same, but on some occasions works slightly differently.