The jdoc syntax is Joomla's way of being able to include extra functionality easily into your templates without the need for having to write new PHP code. For example the following jdoc command:
<jdoc:include type="module" name="mainmenu" title="Main Menu" />
Would create a menu module for you to output content, the way Joomla uses this syntax just makes it easier to include things rather than having to use raw PHP code.
For a more in-depth explanation on the process Joomla uses to process jdoc commands you can check out Joomla advanced topics | how are templates executed
Please let us know if you had any other questions.