It's a family of microcontrollers by Espressif. A microcontroller (MCU) is a type of integrated circuit (IC: physical appearance: Black rectangle of epoxy with exposed metal tabs). It has, in the same IC, a CPU, RAM, non-volatile FLASH memory, and a collection of peripherals which vary depending on the MCU. These usually include I/O protocols, ADCs, DACs, DMA controllers, math processors etc.
Stated another way: An MCU, like the ESP32, is a tiny computer, sans the hardware you physically interface with it.
The computing power, and memory is much lower than your desktop or laptop PC (or mobile phone), but if you are using it to run a dedicated task, instead of using a big OS like Linux or Windows, it can complete the tasks really fast (often microseconds) and with minimal power use. This is because it's easy to program to do exactly what you need, with nothing competing for the hardware.
A note on ESP in particular compared to other MCUs: It's one of the only (Or was?) options which has Wi-Fi integrated into the MCU itself. It's a good default if you want that.
Stated another way: An MCU, like the ESP32, is a tiny computer, sans the hardware you physically interface with it.
The computing power, and memory is much lower than your desktop or laptop PC (or mobile phone), but if you are using it to run a dedicated task, instead of using a big OS like Linux or Windows, it can complete the tasks really fast (often microseconds) and with minimal power use. This is because it's easy to program to do exactly what you need, with nothing competing for the hardware.
A note on ESP in particular compared to other MCUs: It's one of the only (Or was?) options which has Wi-Fi integrated into the MCU itself. It's a good default if you want that.