One or more operations are either performed all and the process will not be interrupted by any factors, or they will not be executed.
Example:
x = 10; //atomicity
y = x; //two operations: read x and assign x to y
x++; //three operations: read x, plus 1 and write new value
When multiple threads access the same variable, one thread modifies the value of the variable, and other threads can see the modified value.
The order in which the program is executed according to the order of the code.