trim in javascript ? what this code is doing?

I was looking for a trim function in JavaScript which doesn't exist and some code on Googling suggests that use:

function trimStr(str) {
  return str.replace(/^\s+|\s+$/g, '');
}

I want to know how str.replace(/^\s+|\s+$/g, '') works. I understand that this is some form of regular expression but dont know what it is doing.