Board Thread:Support Requests - Getting Technical/@comment-45486825-20200930163048

When looking into templates' source code, I see triple curly brackets often, especially with numbers like so:

User:

That template is a template for usernames, formatted as, that makes it easier to link usernames without having to do USERNAME

Is someone able to explain this to me, or link me to a MediaWiki article or something of the sort that can? 