Transmission diagnosis will be required when your vehicle displays an automatic transmission warning light or message on the dash. The warning means the transmissions computer has identified a problem with the electronics or the transmission has an internal mechanical problem.
When a code sets the transmissions computer will react in a certain way depending on the type of failure. Sometimes the computer will stay in one or two gears. Often it will drive up the internal oil pressure, which will cause the shifting to be very hard. Other codes will cause the automatic transmission warning light to be on but the transmission will shift fine.
Either way the best thing to do is get it checked out as soon as you can. If the shifting feels harsh and jerky or is not functional it would be best to not drive the vehicle until you talk with your transmission shop.