What is the difference between distance and displacement?
Distance is a scalar quantity and displacement is a vector quantity.
Distance has only magnitude. It measures the actual ground covered. Distance can only be positive.
Displacement is measured with reference to a specific point. It is a straight line from the starting point (origin) to the end point. It is therefore also the shortest distance between two points. If the displacement vector is away from the reference point (aka datum) it is often defined as positive and if toward it is defined as negative.
It is possible to have an average displacement equal to zero if the object starts and ends in the same position.
Displacement is used in considerations of oscillations (including waves). The reference point is the equilibrium position for the oscillator. Displacements above the equilibrium (if above is appropriate) are positive and below the equilibrium are negative.