A command block is a block (basic unit of building in the game Minecraft™) that can execute commands. Because it cannot be obtained or edited in Survival mode without cheats, it is primarily used on multiplayer servers, in Creative worlds, and custom maps.
An impulse command block (appears orange-tan-ish) is the default block type; it executes only once when activated.

A chain command block (appears green) executes every time when triggered.

A repeating command block (appears blue) executes every game tick as long as it is activated.

(Extracted from https://minecraft.fandom.com/wiki/Command_Block)