• It is a function which does not change any global state of a blockchain, which reduces the gas.
• It depend on the function argument and modify the function local variable and does not deal with global state variable and external contracts.
• e.g,

function addNum(uint _a, uint _b) public pure returns (uint){
	returns _a + _b;
}

here the fn addNum depend only on the input parameters and does not deal with the blockchain state so it is a pure fn.

Note:

If the function returns the result from input parameter only but has a modifier which depends on the state variable then the function is not a pure function.