Rowena,
Basically California has the strictest emissions standards in tthe US. Each year when you go and get a new inspection sticker, your car must pass the emision test. The mechanic sticks a special guage in the tailpipe to measure the polutants comming out and it is too high, you cant get an inspection sticker (and cant legally drive) until you bring it in and get the catylitic converter (or whatever) fixed or replaced.
Its a good ideea, but it does put a strain or poorer people who cant afford to fix up their old cars. Even though, I think it is a good idea and it is forcing car companies to create cleaner cars with better gas mileage.