Initializing values by memsets

I have used an array as dt[100]; then,i have initialize the values like memset(dt,63,sizeof(dt)); then i have seen the value assigned in those array like 4557430888798830399.can i have the explanation?

#include <bits/stdc++.h>
#define ll long long int
using namespace std;
ll dt[100];
int main()
{

 memset(dt,63,sizeof(dt));
 cout<<dt[0]<<endl;

}