To activate "Development mode" [A], go to the game setting and activate it on the "General" page first. This will unlock an additional menu [B] in your game settings where all further options [C] can be accessed.
Activating dev-mode also enables the "dev-bar" [D-I] on the top of your screen, which will show different options, depending on your location in the game - like the main menu, colony- or world-map.
Enable test map sizes
Verbose logging
[OFF] - Only situationally useful, keep off unless you know what you're doing.
Auto-reset mods config on crash
[OFF] - Causes more harm than not, always keep off!
Quick start has no cryptosleep sickness
Start game with dev palette on
Mostly useful for modders and for testing. Just makes it the dev palette is already opened after loading into a map, which saves you one mouse click.
Open log on warnings
[OFF] - Usually (yellow) warnings are nothing the average user has to care about, so keep that off to avoid your in-game log popping up every other second.
Close log by pressing cancel
Permanently disable development mode
Never disable dev-mode - simple as that!
[D] - Debug Log
[E] - Tweak Values
[F] - Dev Palette
[G] - God Mode
[H] - Debug Actions / Settings / Output
[I] - Inspector
The in-game log directly shows the last 300 lines of logs you can found written in the Player.log-file. Selecting a line will show additional info (stacktrace) - white lines are general informations, yellow lines typically warnings about something not being as the game or a mod expects it - red lines are actual errors.
Debug Actions/Settings/Output
Those three pages contain any base game or modded dev-actions. Clicking the pin besides any entry will 'pin' it to the dev-palette, so it can be accessed without opening this menu each time. Actions here are mostly meant for game-testing and ideally shouldn't be used for regular gameplay - not everything here is safe and can at times break your game. If you know what you're doing though, you won't need this description & warning anyways...
All pinned features will appear here to be directly accessed without having to open the respective menu each time. Mostly something modder & tester will really care.
The Inspector tool lets you view additional info based on your mouse pointer location. The most common use is to find and identify otherwise invisible things on your map. Just hovering over a tile shows you all things located there with additional data about it.
One of the most useful info might be the exact map coordinates an item - or the mouse pointer - is located at. Like when an error references a ticking item with its position, to easily find it on your map.