Explain this javascript function to me

I was playing codewars, i came up with this
function nbDig(n, d) {
    // your code
    var count;
    count=0;
    while(n>0){
      var squared = (n*n).toString();
      var regex = new RegExp(d,"gi");
      var matches = squared.match(regex);
      if(matches){
            count = count + matches.length;
        }           
      n--;
    }
    return count;
   
}

However one of the best practices submitted was the following below

function nbDig(n, d) {
var res=0;
    for (var g=0;g<=n;g++){
      var square=(g*g+"").split("");
      square.forEach((s)=>s==d?res++:null)
    }return res;
}

I do not understand the (s)=>s==d

I believe that this is function(s){ s==d?res++:null}

Is that correct, can someone provide some good documentation or link for the (s)=> paradigm in javascript?