This seems to best explain it that i found on StackOverflow website for formatting in Python.
Format strings contain “replacement fields” surrounded by curly braces
{}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling:{{and}}.
My guess is that formatting syntax will be similar for other languages as well as i think GH is written in dot net. I originally found out about double brackets when i was trying to format paths for split tree so i needed the curly brackets.