This site is designed to help you to learn how to use PHP. It does not give you all the information you need to use every bit of PHP. Once you start creating your own pages you will need to look up extra information. Use php.net.
The only bit which really matters is the search field at the top right. Type echo into it and press enter on your keyboard. Now the first thing to look at is the examples so scroll down to see them. Once you sort of understand you should probably come back to the top of the page and read the Description bit but that may not make sense until you see the examples. As you can see there is a lot to learn about echo but you will not need to know it yet.