An urn is a hypothetical container from which one may draw one of a number of objects (usually defined to be balls) of assorted colours.
The idea is that one cannot see the ball one is withdrawing until it is out of the urn.
Therefore it is not possible to know in advance what colour ball one is taking.
The probability that a particular ball being withdrawn has a particular colour depends entirely on the distribution within the urn of the various colours of the balls within.
This is used as an abstraction of a randomizing device in probability theory.