Check engine

I have a 1999 lexus es300 with 80,000 miles and a v6 3.0L engine. The check engine light came on and I took it to a mechanic to get a diagnostic and he said one or two or three of the sensors will need to be changed. He was saying oxygen sensors. Is this a big problem. Is is something that needs to be fixed immediatley? What are the effects if not fixed too soon. And also what can go wrong?