It is a unary operator which is used in finding the size of data type, constant, arrays, structure etc. For example:
#include <stdio.h>
int main(){
int a;
float b;
double c;
char d;
printf("Size of int=%d bytes\n",sizeof(a));
printf("Size of float=%d bytes\n",sizeof(b));
printf("Size of double=%d bytes\n",sizeof(c));
printf("Size of char=%d byte\n",sizeof(d));
return 0;
}