The document discusses buoyancy and the buoyant force. It defines buoyancy as the tendency of an object to float when in a fluid. The buoyant force is the force in fluids that acts against gravity. It explains that an object floats if the upward buoyant force is greater than the downward force of gravity. Salt water provides greater buoyancy than fresh water because salt particles also push up on the object in addition to water particles. Examples where buoyancy is important include the Plimsoll line on cargo ships and how hot air balloons are able to rise into the air.