not only does God play dice with the universe,
he throws them where they cannot be seen
Stephen Hawking in reference to
quantum mechanics and
black holes
On the whole Einstein thing,
a better translation form the German would
be
God is slick but he is no malicious.
It is about the physics as
hramyaegr says,
but it was not due to
a
lack of knowledge surrounding the subject at the time.
It was a statement of Einstein's disagreement
with the proababilistic nature of quantum mechanics.
He held to his point of view, that the world was determanistic
and that we didn't have a good enough theory to
realise this, right untill his death. By the
time of his death quantum mechanics was quite well
understood.