What does python zip(*X) do with "*"(asterisk)?

Question

E.g, I get a matrix

>>> matrix=[[1, 2, 3], [4, 5, 6]]

I can transpose it with:

>>> zip(*matrix)
[(1, 4), (2, 5), (3, 6)]

I tried to use without it, but failed:

>>> zip(matrix)
[([1, 2, 3],), ([4, 5, 6],)]

What does the "*" mean here?