Developer Mode adds extra options to Discord's menus — most importantly, the ability to copy IDs. Moderators and reporters rely on IDs because usernames and display names change; a numeric ID does not.
On desktop or in a browser, open User Settings (the gear icon), go to Advanced, and switch Developer Mode on. On mobile, tap your profile picture, open App Settings, then Advanced, and enable Developer Mode there.
Once it is active, right-click menus on users, messages, and channels will include options like Copy User ID, Copy Message Link, and Copy Channel ID. If those options are missing, Developer Mode is still off or the app needs a restart.