This is a surprisingly tough question because there are a huge number of things that can cause conjugated hyperbilirubinemia (i.e., high bili in blood + urine). In general, it's caused by one of two processes: your liver could be having trouble secreting bile (a viral infection, a drug reaction, or a congenital condition could cause this. It can also happen during pregnancy). Alternatively, the bile could be getting stuck on its way to your intestine (gallstones can do this by plugging up the ducts).
When they do the ultrasound, they'll be looking for evidence of obstructed bile ducts -- areas where the ducts look constricted, or areas where they look dilated (which suggests that bile is getting dammed up by something). They'll check your gallbladder and see if there are any pesky gallstones lying around, and they'll also be looking at your liver to see if anything seems amiss there.
If you have more specific questions, please ask -- I may be able to point you towards some information.