Final answer:
Bursae are fluid-filled sacs that prevent friction at joint areas by separating adjacent structures and preventing direct rubbing.
Step-by-step explanation:
Bursae are fluid-filled sacs that serve to prevent friction between skin, muscle, or tendon and an underlying bone. They are located in regions where skin, ligaments, muscles, or muscle tendons can rub against each other. Bursae reduce friction by separating the adjacent structures, preventing them from rubbing directly against each other.