Complex sentence: A microcontroller (or MCU, short for "microcontroller unit") is a small computer (SoC) on a single integrated circuit that contains a processor core (s), memory and programmable input/output peripherals.
Split into simpler sentences: A microcontroller (or MCU for "microcontroller unit") is a small computer on a single integrated circuit. In modern terminology, it is a System on a chip or SoC. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals.