What does it mean when I get the message "too many values to unpack (expected 2)"

# The dictionary will be something like:
# {'Jason Seifer': ['Ruby Foundations', 'Ruby on Rails Forms', 'Technology Foundations'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Often, it's a good idea to hold onto a max_count variable.
# Update it when you find a teacher with more classes than
# the current count. Better hold onto the teacher name somewhere
# too!
#
# Your code goes below here.

def most_classes(teacher):
    max_count = 0
    busy_teacher = ' '
    for k, v in teacher.items():
        if len(v) > max_count:
            max_count = len(v)
            busy_teacher = k
    return busy_teacher


def num_teachers(teachers):
    return len(teachers)

def stats(teachers):
    teacher_name = ' ' 
    num_classes = 0
    teacher_class_num = []
    for teacher, classes in teachers.items():
        teacher_name = teacher
        num_classes = len(classes)
        teacher_class_num.append([teacher_name, num_classes])
    return teacher_class_num

def courses(teachers):
  courses = []
  teacher_classes = []
  for teacher, classes in teachers:
    for each_class in classes:
      teacher_classes.append(each_class)
      for one_class in teacher_classes:
        if one_class in courses:
          continue
        else:
          courses.append(one_class)
  return courses