function(taskListItem, checkBoxEventHandler) How come "taskListItem" and "checkBoxEventHandler" work?

function completedTasks(){
//here function completed tasks
}

function incompletedTasks(){
  //here is function incompleted tasks
}
/*function is callled from looping at the end of code*/  
function setEvent(liItems,checkboxHandler){
 //assigning buttons (edit,delete) or checkbox to variable 
    var edit_btn = liItems.querySelector("button.edit");
    var delete_btn = liItems.querySelector("button.delete");
    var checkbox = liItems.querySelector("input[type=checkbox]");

  //making it work
    edit_btn.onclick = editTask;  //when it is clicked editTask function will execute
    delete_btn.onclick = deleteTask; //when it is clicked deleteTask function will execute
    checkbox.onchange = checkboxHandler; /*and when you make any type of changes i.e. uncheck or check in     
                                                                            checkbox the checkboxHandler(i.e. completedTasks or incompleted    
                                                                            task) function will be executed */  

}
//here we're accessing all the items or elements inside incompleted tasks list
for(var i=0;i<tasksLeft.children.length;i++){
      //looping through each item of our taskcompleted section
      setEvent(tasksLeft.children[i],completedTasks);
/* calling a function setEvent which is in your case bindTaskEvents and passing two values (argument)to it first is incompleted tasks[i] item and second is function completed task */
  }
  for(var i=0;i<tasksComplete.children.length;i++){
      //looping through each item of our taskcompleted section
      setEvent(tasksComplete.children[i],incompletedTasks);
/* calling a function setEvent which is in your case bindTaskEvents and passing two values (argument)to it first is completed tasks[i] item and second is function named incompleted task */
  }