If you want to insert a wiki page into another page you use the InsertPage plugin. The following code demonstrates the basic technique. Of course, first you need the page to be inserted; we'll use InsertPageTest.

This

Here, I insert the text of another page:\\
[{InsertPage page=InsertPageTest}]

produces this

Here, I insert the text of another page:
There is no page called 'InsertPageTest'. Would you like to create it?

If you want to insert a section of a page into another page use a horizontal ruler (four dashes, '-') to part sections. The page Wiki.How To.Insert Pages Test Section contains three sections that is included below:

This

Here, I insert section 3 of another page:\\
[{InsertPage page='Wiki.HowTo.InsertPagesTestSection' section='3' }]

produces this

Here, I insert section 3 of another page:

Cyclical dependencies are checked for, so inserting a page into itself won't work.