What does [] means?

In this mission:

https://py.checkio.org/ru/mission/unix-match/

There is told that [bla bla bla] - means any character in sequence.

But what does mean empty brackets? []

It does not mean any character and it does not mean no characters, so, what does it mean?

print(unix_match("nametxt","name[]txt"))  # == False

print(unix_match("name.txt","name[]txt"))  # == False

Thank you a lot.