QMK has a staggering number of features for building your keyboard. It can take some time to understand all of them and determine which one will achieve your goal.
Advanced Keycodes - Change layers, type shifted keys, and more. Go beyond typing simple characters.
Audio - Connect a speaker to your keyboard for audio feedback, midi support, and music mode.
Auto Shift - Tap for the normal key, hold slightly longer for its shifted state.
Backlight - LED lighting support for your keyboard.
Bootmagic - Adjust the behavior of your keyboard using hotkeys.
Dynamic Macros - Record and playback macros from the keyboard itself.
Key Lock - Lock a key in the "down" state.
Layouts - Use one keymap with any keyboard that supports your layout.
Leader Key - Tap the leader key followed by a sequence to trigger custom behavior.
Macros - Send multiple key presses when pressing only one physical key.
Mouse keys - Control your mouse pointer from your keyboard.
Pointing Device - Framework for connecting your custom pointing device to your keyboard.
PS2 Mouse - Driver for connecting a PS/2 mouse directly to your keyboard.
RGB Light - RGB lighting for your keyboard.
Space Cadet - Use your left/right shift keys to type parenthesis and brackets.
Stenography - Put your keyboard into Plover mode for stenography use.
Tap Dance - Make a single key do as many things as you want.
Terminal - CLI interface to the internals of your keyboard.
Thermal Printer - Connect a thermal printer to your keyboard to be able to toggle on a printed log of everything you type.
Unicode - Unicode input support.
Userspace - Share code between different keymaps and keyboards.