Hello I am new to plone. I want to learn plone and I have a task to make page template but I couldn't understand what exactly are they and what do they do and why are they build. What should I know before starting making page templates
First, you must to learn how to create a Plone product. You can use mrbob for this: https://github.com/plone/bobtemplates.plone
After, you must create a custom view (http://docs.plone.org/develop/plone/views/browserviews.html#creating-and-registering-a-view) then your page template: http://docs.plone.org/adapt-and-extend/theming/templates_css/template_basics.html
Maybe you could describe what you want to do
Page template could be 'anything', do you mean layout, content.
There are also a lot of add-ons that you often can use instead of 'making something new'