What does unhashable type mean?

Answer 1

This means that you cannot use a variable storing a list or dictionary when declaring a dictionary.
Example:
alist = [1, 2, 3]
adict = {akey: alist}
This code will cause an error due to the reason above.

Answer 2

"unhashable" means it cannot be used to build hash.

Dictionaries use hash-functions to speed up access to values through keys