As you saw with Play1/T1, it printed the text 'Hello World' where it was used. You also already know that {{T1}} is what calls the template/places it on the article. However, what's really going on? What do all those symbols mean?

Both [[ ]] and {{ }} are programming calls, basically attempts to access other articles or functions/procedures.

[[ ]]

The use of the brackets [[ ]] makes an attempt to identify if what is being called is an article. If so, it attempts to create an HTML hyperlink to it. However if the link provided is prefixed with a Category, it instead adds the target as a category to the current page. If the target is a file, it attempts to place that file on the current article instead. You may get around these "exceptions" and keep the original hyperlink functionality by appending a colon : at the start of the text. ie; [[:File:image.png]]

{{ }}

This markup language is the bread and butter that makes all the dynamic magic happen. These curly brackets attempt to "include" the article between them.
If there is a template, "Template:Test", {{Test}} as well as {{Template:Test}} will work for including that template no this page. In the first example though, if the template 'Template:Test' did not exist, but the article 'Test' did, {{Test}} would instead "include" the article Test.
Of note; if both Template:Play1 as well as the article Play1 exists, {{Play1}} would by default use the template.
These curly brackets also allow the call of wiki-specific methods, such as #if and switch. These will be covered later.

{{{ }}}

Within a template or other piece of code that is meant to be included, ie, {{ }}, you may see the use of triple curly brackets. These are different from including pages (double-curly brackets), because the text used within it is not used to look up an article / it does not "include" another existent page. What the triple brackets do, is look up information that may have been passed to it when the article was "included." Or rather, this is called a parameter.
{{{1}}} - When a number is used, wikia looks for a passed parameter according to the unlabeled position it was sent in. Example: {{Test|Crash|dummies}} - in this case, on the article/template, 'Test', {{{2}}} would return "dummies".
{{{nation}}} - When text is used instead of a number, wikia looks for a passed parameter by that name. The position is irrelevant. Example: {{Test|nation=fire}} - This would instruct the page, 'Test', to use a parameter named "nation" with the value "fire" applied to it.