Need help understanding this basic JS concept regarding objects and properties

Question

Since myObj is not a part of the checkObj function, how can checkProp contain myObj's properties/values when you call myObj[checkProp]? I'm new to learning JS and this is going a bit over my head.

var myObj = {
  gift: "pony",
  pet: "kitten",
  bed: "sleigh"
};

function checkObj(checkProp) {
  if(myObj.hasOwnProperty(checkProp)) { 
    return myObj[checkProp]; 
  } else {
    return "Not Found";
  } 
}

checkObj("gift");