What are references and strings?
Let's start with the basics:
References are links to rows or people and therefore have all sorts of their own metadata. They also have a name based on the Display Column.
Strings are a set of characters, like the name or status of a task, or the cost of an item on your grocery list.
This is how they look in practice:
"This is a string" is just text in the canvas. "This is a reference" is a link to a row in a table elsewhere in your doc. When you type an @, you can place a reference to both People who are collaborating on your doc, as well as to rows in tables. The example above is referencing the only row in the table below:
The text that you see in the reference is actually the identifying characteristic of the row, or the Display Column. Here's what the same reference looks like when I change the Display Column:
You might also be interested in the following help article: Add a reference to a page, table, row, or person