What are front axle boots. How serious is it when they are torn? on 2001 BMW 325i

Both front axle boots are torn. The cost quoted to repair was $413.71. What does it mean that they are torn?

Asked by for the 2001 BMW 325i
The axle boots are part of the axle shaft assembly - please visit the following link for more info -, There are two boots on each axle, the cover the "joints" that allow the axle to function while the drive wheels are moving up and down over bumps and turning. The boots can tear because of aged or road debris. A torn boot will allow the grease that keeps the joint lubricated to leak out and dirt and moisture to get in, damaging the joint.