The large particle represents a pollen grain suspended in water, as first observed by Robert Brown in 1827. It is being struck constantly and from all directions by water molecules that are far too small to see under a microscope.
At any instant the bombardment is not perfectly balanced: slightly more molecules hit one side than the other. These random, unbalanced collisions make the grain jiggle along an erratic, unpredictable path. Untick Show molecules to see exactly what Brown saw — a grain moving for no visible reason.
Try raising the temperature. The molecules move faster (average kinetic energy is proportional to absolute temperature), so they strike the grain harder and more often, and the jiggling becomes more violent. Try a smaller grain: fewer collisions per instant means poorer averaging, so the imbalance is proportionally larger and the motion is more pronounced.
Einstein's 1905 analysis of this motion, confirmed experimentally by Perrin, provided decisive evidence that matter really is made of molecules in ceaseless random motion.