This is a default sample page. For pages like this, choose the “Default layout” option to the right under the template drop down. You will then be able to control the content via the “Template: Flex content” block that you will find below.
If you want to just display the content that is entered here in a similar fashion to typical default page, enter whatever you like here and choose the “Page Content” option on flex content.
Other than that, you have several other options to choose from for adding flexible content components.