Entropy is a difficult concept with which to grapple. The following example may help. I place a dozen marbles in the left hand, and shake the box. When I look into the box again, the marbles are now distributed between both halves, provided the box is level. Moreover, the probability that this will occur decreases rapidly the more marbles are placed in the box.
Molecular systems are different because the number of particles is so large.
We cannot observe each individual molecule, and are obliged to study properties such as pressure, which are average properties to which all the molecules contribute. Average properties are influenced by the entrophy of the whole system, and disorderliness is more probable than order. No one has ever seen an empty balloon spontaneously rush around the room, gathering up gas molecules and inflating itself. According to Newtonian mechanics, there is no reason why this should not happen. Entropy imposes the arrow of time upon the system.