The Custom module lets you write your own content for a template position using the site's editor, in the same way you would write an article. It is the usual choice for anything that does not fit an existing module type — a short welcome message, a block of contact details, an image with a caption, or a promotional panel.
Because it accepts arbitrary markup, it is also the simplest way to place bespoke HTML into a template position without building a custom extension. Site modules documentation