It means that the snipppet produces its output by setting placeholders and rendering a tpl that contains those placeholders.
Say you have a snippet that gets information on a particular user from the DB and puts it on the screen.
The tpl chunk might look like this:
<div class = "user_info">
Name: [+user_name+]<br />
Address: [+user_address+]<br />
Email: [+user_email+]
</div>
If you wanted to change how the output looked, you’d create your own tpl chunk, let’s call it "MyTpl." You could add a phone number or some other data available in the snippet. You could also change the HTML and the class names. Then you’d just pass the name of that chunk to the snippet:
[[SnippetNamet? &tpl=`MyTpl`]]
The snippet will replace the placeholders (the things between the + signs) it recognizes and send the result off to the browser.
This lets you have great control over the visual presentation of the snippet’s output without touching the code of the snippet or worrying about your work being destroyed when you update the snippet to a new version.
