Piece of the front suspension which consists of a bearing upon which the wheel turns.
In newer cars this piece is sealed (ostensibly to prevent contamination, but I have my doubts), requiring replacement of the whole thing when it wears out, but in older cars, this was a real honest-to-$DIETY bearing, that you could take out and grease, which is referred to as packing the bearing.
As your front wheel bearings wear out, the front wheels will start to wobble, causing advanced tire wear, and requiring minor, frequent steering corrections. This can be dangerous, because as the bearings degrade, they begin to produce more and more heat, eventually seizing to the race and locking up the wheel.