Can someone comment the stuff this C code does?

#include <stdio.h>

int main() {

char frase[100];
scanf("%[^\n]s",frase);

int i,contnum=0,contlet=0;
for(i=0;frase[i]!='\0';i++) {
if(frase[i]=='@'){
frase[i]='\0';
break;
}
else if(frase[i]>=48 && frase[i]<=57){
frase[i]='*';
contnum++;
}
else if(frase[i]>=97 && frase[i]<=122){
frase[i]='_';
contlet++;
}
}

printf("%s\n",frase);
printf("Total numericos substituidos: %d\n",contnum);
printf("Total alfabeticos substituidos: %d\n",contlet);
}