this function for taking input from string how does it take input using bitwise operator and manipulat string into no please explain

this function for taking input from string how does it take input using
bitwise operator and manipulat string into no please explain

How has the string been manipulated to integer and what is use of bitwise
operators..this function has been used in c to take input from string of
numbers
inline LL inp()
{
LL num = 0;
char p = gc;
while(p<33)p=gc;
while(p>33) {
num = (num << 3)+ (num << 1)+ (p -'0');
p = gc;
}
return num;
};