903 is a symmetrical 4 object siteswap where three objects are juggled in a high cascade at about seven times the height of the basic pattern, while the other object is thrown from hand to hand at about half the height of the basic pattern. One hand throws a 9, then after pausing for one beat the same hand throws a 3, then the same throws are repeated on the other side. The rhythm of the pattern is right-pause-right-left-pause-left.
This is an excited state siteswap, starting in the state 1011001. You can do this pattern from a cold start by leaving out the 3 in the first round, or you can go from the basic pattern to 903 using transition 557, and back to the basic pattern with the transition 142:
Before learning this pattern, you should already be good at 900, 603, 933333 (starting from both sides), and 750.