This part is called a ball joint boot. Ball joints are part of your suspension and allow your front wheels to move up and down and turn left and right. This boot covers and protects your ball joint.
Your suspension is, of course, exposed to nasty stuff getting kicked up from the road. Stuff like rocks, salt, water, dirt, and chunks of asphalt. If all that debris were constantly hitting your ball joints, they wouldn't last very long. So, someone thought, "Hey, why not include protective ‘boots’ that go over the ball joints?" Great idea. And they work… for a while.
Those boots not only protect against debris but also against heat and friction. So, they must be flexible. In addition to keeping contaminants out of your ball joints, they also keep lubricants in and allow the hot grease to expand. This is vital to make sure metal parts are moving smoothly.
Sometimes the ball joint boots get damaged after constant assault by road junk or temperature extremes. Other times they just get old. Rubber and other flexible materials can eventually just wear out (think of old rubber bands that break because they're brittle from age). A regular suspension inspection will assess the condition of your ball joint boots. When your NAPA AutoCare Service Advisor says there's a problem with one of your ball joint boots, heed the warning. If they have a tear in them and aren't replaced, lubricant can escape the ball joint and debris can get in and start creating a destructive mess.
Also, keep in mind that if a ball joint is bad on one side, the other side might be on the verge of going, too. Replacing both ball joints and ball joint boots at the same times just makes sense.
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