The GTK Inspector is similar to a web browser's inspector, but for GTK apps. Using the Inspector can greatly speed up development, and allows you view and to test out changing properties without recompiling an app. You can also test out temporary in-app CSS.
First, make sure you have elementary-sdk installed. Then enable the Inspector keybinding:
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
Focus an app, then launch the Inspector by pressing Ctrl+Shift+I to inspect the widget beneath your cursor, or Ctrl+Shift+D to open the inspector without a widget selected.
You can also run it temporarily together with an app by running: