What does the L at the end of a number do?

Question

For example:

int one = 20;
int two = 20L;

cout << "one: " << one << " two: " << two << endl;
// one: 20 two: 20

cout << "one: " << sizeof(one) << " two: " << sizeof(two) << endl;
// one: 4 two: 4