WikiTree uses the MediaWiki software platform, the same one behind Wikipedia. MediaWiki uses markup language to display text with different formatting, like bold, italics, headers, hyperlinks and more.
Their are many, many markup languages, but they all have similarities. If you've ever used a modern word processor, like Microsoft Word, what you are seeing uses a markup language. The language for the internet, HTML, is a markup language.
The references are part of MediaWiki. Here is a description from Wikipedia: https://en.wikipedia.org/wiki/Wikipedia:Citation_templates
Basically, on every page, you enter a source citation between two tags. These are markup tags, just like in HTML. The beginning tag is "<ref>" and the ending tag is "</ref>." Your citation goes between these.
So, if your citation was this:
"Locate a Loved One," database, ''Catholic Cemeteries Brooklyn'' (http://www.ccbklyn.org/information-news/locate-a-loved-one : accessed 23 August 2020), interment, Michael Pierce, 1 October, Holy Cross Cemetery, St. John the Baptist, row M, plot 4.
Then the citation wrapped with markup for a reference, would be this:
<ref>"Locate a Loved One," database, ''Catholic Cemeteries Brooklyn'' (http://www.ccbklyn.org/information-news/locate-a-loved-one : accessed 23 August 2020), interment, Michael Pierce, 1 October, Holy Cross Cemetery, St. John the Baptist, row M, plot 4.</ref>
Now, the magic. At the bottom of every profile is the sources section that looks like this:
== Sources ==
That references tag is basically telling the software to output all the citations that are wrapped in reference tags in a nicely formatted bibliography, cited works or whatever you want to call it.
There are also ways to give a reference a name and reuse it when multiple pieces of information came from the same source. See that page I linked to Wikipedia.
I highly recommend starting here: https://www.wikitree.com/wiki/Help:Biographies and working your way through the help guides, but you can check this page out for another explanation on the references: https://www.wikitree.com/wiki/Help:Sources#Embed_them_as_references_.28footnotes.29
You can also Google markup languages and read through guides at Wikipedia. A great learning guide for HTML is here: https://www.w3schools.com. You will see wiki markup, HTML and CSS on WikiTree pages, so you'll benefit from a basic understand of the concepts.