What does the check light mean on the dash of a 1998 Camry?

The engine control computer has detected a problem. The kind of problem can be found out only when the computer is put into diagnostic mode by someone who knows how to do this. Get the vehicle inspected by a mechanic who has the documentation for your model (or go to an authrorised Toyota dealer's service centre) ASAP as the problem could be serious and lead to engine damage if not repaird. Quite often, however, it's just a failed O2 (oxygen) sensor.