Structure Mode is a brand new way to write Semgrep rules that guides users via UI as opposed to requiring them to write YAML. Structure mode makes rule-writing easier for inexperienced rule-writers, but it also adds cool new features for seasoned rule-writers that should speed up their workflows as well.
Structure Mode replaces the now deprecated "Simple Mode", as it offers more robust functionality paired with an intuitive interface that's just as easy (if not easier) to understand than Simple Mode.
To learn more about Structure Mode, read our blog post which outlines all of the shiny new capabilities in detail.