HDD Access Light
First of all, you should identify the light you're looking at. The newer PS3 Slim models have a single LED it uses near the power button, and it has a couple of smaller lights near the USB ports on the front left. Older PS3 units have two main LEDs near the power button and the same smaller LEDs on the left. If you're seeing a yellow light from the smaller LEDs near the USB ports, then you have nothing to worry about. That light is only the hard drive disk (HDD) indicator light, which means the PS3 is reading its hard drive, a perfectly normal procedure. As long as the main LED near the power button isn't yellow, your PS3 is probably working properly.
"Yellow Light of Death"
Many gamers have termed the yellow LED indicator light as the "yellow light of death." Sometimes the main PS3 LED will alternate from green to yellow to green and finally to a blinking red light, or it may blink yellow then red, or just yellow repeadedly. Regardless, if the indicator light turns yellow, your PS3 has almost certainly experienced a major error and requires service.
Warranty
If your PS3 is still under warranty, your best option is to see if your retailer or if Sony can repair or replace your PS3. Although this may take some time, you won't have to deal with trying to fix it yourself or with voiding your warranty by opening the unit to work on it. You can request support online at service1.us.playstation.com, and you can inquire with Sony support at us.playstation.com/support/ask. If your PS3 is no longer under warranty, you may need to pay someone to professionally repair the unit or you may need to replace the entire console.
Warnings
Many websites provide you with do-it-yourself guides for fixing the "yellow light of death" error on your PS3. If your console is no longer under warranty, working on the problem yourself may seem like a good idea, but keep in mind that you could cause additional damage to your PS3. If your console is still under warranty, keep in mind that opening the PS3 to work on it does void the warranty, which means afterward you won't even have the option of taking advantage of warranty servicing options.