Structural editing commands are like 'slurp' - swallows an expression inside another one; 'barf' - spits out a thing out of an expression; You can also do it from the left or right side. 'wrap' - wraps a selection into an expression; 'unwrap' - does the opposite; 'transpose' - swaps two expressions at point, and there are more commands.
Once you learn the basic structural editing commands, writing code becomes like composing poetry out of haiku pieces. Instead of thinking like: "how do I grab these vars used inside this function and refactor it to be them in their own unit?...", you'd just grab some expressions and move them around, like bricks or lego pieces. It is extremely satisfying way of writing programs. The only drawback of that approach is that later it becomes harder to work with "more traditional" PLs, you just can't easily manipulate code the same way in Python, JS/TS, Kotlin, Java, C++, etc. - you need a "structured", homoiconic, lispy language for that trick to work. Treesitter makes an effort to improve the process, but it still not on the same level of simplicity.
I think they mean when you learn the shortcuts for selecting and manipulating entire blocks between matching parentheses in an editor that helps balance them and so on, making it rather easy to test things out and refactor Lisp-like code.
