The _Site Assets section is a library of re-usable images, text, tables, etc. The children of this page contain information which is included in other pages using the include or excerpt-include macros.
If you want to change the content in the contained pages, be aware that:
Changing page names is problematic. You will need to change all the include and excerpt-include macros manually.
The content is here because it is used in many places. Make sure your change is generic enough to fit the contexts in which the pages are used.
This is a list of all reusable content. If you're not sure where content is used, click on a page and use the (...) > Page Information to view every link to the page.