Anything related to the usage of commands, command blocks and functions in the Minecraft game series. If you're building a redstone device without commands, use the "minecraft-redstone" tag instead. Always use this tag together with the "minecraft-java-edition" or "minecraft-bedrock-edition" tag.
Commands (sometimes also called "cheats", because they are not intended to be used in regular Survival gameplay) are advanced features activated by typing certain strings of text into chat, command blocks or functions. They allow detecting and acting on almost every aspect of the world, from simply teleporting one player to another, to complex datapacks that heavily modify the game's effective behaviour or even bring entire other games into Minecraft.
If it's your first time asking a minecraft-commands question, then please read this Arqade Meta FAQ post first: What to do before asking a minecraft-commands question?
Please use either minecraft-java-edition or minecraft-bedrock-edition together with this tag, depending on which game you play. Not using a tag specifying the game and not mentioning this anywhere else in the question will cause it to get closed as "unclear".
Please also use minecraft-datapack if your question is specific to using commands in functions.
Please only use minecraft-redstone together with this tag if you are really very sure that it is a question about both. Good examples can be found here.
If you want to ask for help about a command or function that does not work as you expected, please make sure to provide a "minimal reproducible example".