What is the point of 'L' here?

What is the point of 'L' here?

//Using the if-else
#include <iostream>
using std::cin;
using std::cout;
using std::endl;
int main() {
long number = 0; // Store input here
cout << "Enter an integer less than 2 billion: ";
cin >> number;
cout << endl;
if(number % 2L == 0) // Test remainder after division by 2
cout << "Your number is even." // Here if remainder is 0
<< endl;
else
cout << "Your number is odd." // Here if remainder is 1
<< endl;
return 0;
}
Here, in the first 'if' condition, why do they have 'L' after 2? taking
'L' out seems to run the code just fine.