Foot bursitis is a condition that involves inflammation and fluid buildup of the bursa sac. A bursa is a small slippery fluid-filled sac; imagine a water balloon that forms over a joint to decrease friction and act as a lubricant between the bones and surrounding skin, muscles, ligaments, and tendons (or soft tissues for short).
The bursa sac consists of a thin-walled membrane (called the synovial membrane or synovium) that produces and contains fluid (known as, you guessed it, synovial fluid). The bursa can become irritated and inflamed due to excessive repetitive joint motion, injury, a bone deformity, and even due to underlying medical conditions like arthritis.