In many cases, different templates across a theme use recurring components such as a header, a navbar or a footer. Those elements can be stored in separate files and included whenever needed. To include such an external snippet, simply wrap its path relative to the current file in statement delimiters.
<@ elements/header.php @>
The example above would simply include the template snippet file
elements/header.php. The file path is always relative to the including file, also in case the including file is included itself. The Standard themes as well as the theme skeleton provide good example for how to split templates into multiple files and keep theme organized.