Key is a variable, The insertion order preservation nature of dictionary objects, so when an user iterate through dictionaries using some syntax like for or in it iterates over the keys.
For different python versions their exist different syntax:
For Python 2.x use:
for key, value in d.iteritems():
for Python 3.x use:
for key, value in d.items():
You can also refer to Python documentation for assistance.
To know more about this you can have a look at the following video tutorial:-