Atmega 328P in Arduino
A CPU or MCU uses very basic instructions which are executed.

If you write code in a program language (like in Arduino IDE), the sketch is converted into many instructions (by the compiler and linker).

The instruction set can be found here.

An example is:

ADC Rd, Rr: Adds two registers and the contents of the C flag and places the result in the destination register Rd.

A CPU/MCU uses registers, which can be seen as very fast 'variables', mostly there are quite few of them, and all operators (like +, -, *, /) are carried out using these registers.

If you are just starting Arduino, it's not so useful to dive into this, the compiler/linker will take care of these 'details'; just be glad you don't have to write in assembly (anymore).