Simply put, dune beaches are beaches with dunes on them.
Mountains of shifting sand are called dunes. Wind will kick up grains of sand on the beach and as the grains fall back to Earth, they dislodge more sand, and on and on.
When the moving mass of sand meets an obstacle—a plant, tree, or fence—it can go no further. The wind does not stop, however, so the sand forms a slope against the barrier. As the wind continues, sand is blown up the slope and over the top, or crest.
Suddenly it is sheltered from the wind by the slope it just surmounted, so it falls, ultimately creating another slope on the other side.
As the process of sand blowing over the top doesn’t stop, the mountain of sand, or dune, gradually progresses over the land.