I was just told that my airconditioner is missing a j trap. What is it and is it easy to replace?

Who told you that and didn't tell you what it was?

It's a droop in your condensation drain when the water exits your evaporator coil. It is needed if you have a heat pump for it to drain right and you add a breather Tee after the trap. It is not needed for draining purposes on a coil that sets above the blower(gas furnace)for it to drain right. Unless code ask for it.