The Brownian Motion focuses on the free motion (with help of microscopic observation) of extremely tiny particles of solids which are lying in liquid media unable to dissolve them.
The very experiment took place during biological research made by Mr. Brown. However, it wasn't accepted for quite some time as scientists where looking for a more solid data regarding the Atomic Theory itself (early XX century). Finally though, scientists understood that those facts are closely related to the very nature of matter, e.g.
INTIMATE NATURE OF LIQUID MEDIA
thanks to which, we can state that:
→ liquid media are made by molecules
→ those molecules are in state of constand, random movement which can be further stimulated by temperature. It confirms the theory of Energy Distribution in molecules which appeared at the end of XIX century.