What does array[i][0] mean?

Given an array of strings of the same letter type. Return a new array, which will differ in that the length of each element is equal to the average length of the elements of the previous array.

function averageLength(a) { 
  var arr=[];
  var avg=0;
  for (var i=0; i<a.length; ++i)
    avg+=a[i].length; 
  avg/=a.length;
  avg=Math.round(avg);
  for (var i=0; i<a.length; ++i)
  {
    var c='';
    for (var j=0; j<avg; ++j)
      c+=a[i][0];
    arr.push(c);  
  }
  return arr;
}

Whtat does this part mean c+=a[i][0]and where we use j ? Thanks!