a sensor ore modgle or any number of things,your toyota may be running fine but if light is on something is wrong and need to have it scanedto find problem,autozone will do this for free.

Honestly, there are any of about 150 different things it means.  It could be a sensor or module, it could also indicate a problem with the transmission computer components.  95% of these items are emissions related and will affect gas mileage, if not driveability, but a few are directly related to driveability to the point that the car will perform BADLY if they are not working.  Best to have the codes scanned and find out.