Disorder increases in isolated systems
Entropy measures the number of microscopic arrangements consistent with a macroscopic state. A hot gas diffusing into a cold room has trillions of ways to arrange itself, each equally probable at equilibrium. Once mixed, there is an enormous number of arrangements that yield 'uniformly warm room', but practically zero that yield 'hot corner, cold corner' again. The second law states that entropy of an isolated system always increases, reflecting the fact that disordered states are statistically overwhelmingly more likely than ordered states. This is not a constraint imposed by nature but a consequence of probability: disorder is just more common. A broken egg will not spontaneously reassemble because the final 'unbroken egg' state is one specific arrangement out of an astronomically larger set of 'broken' arrangements. The arrow of time emerges from this probabilistic asymmetry.
Entropy and the fate of the universe
In a closed universe, the second law predicts that entropy will increase until the system reaches maximum entropy equilibrium: thermal death, where all matter is at uniform temperature and all useful energy is dissipated as heat. This does not mean the universe is dying now but that, in the distant future, all organized structures (galaxies, stars, life) will have decayed into radiation and cold matter. The expansion of the universe and the second law together imply that the early universe had very low entropy (a highly ordered state), which cosmologists trace to the Big Bang. Some theories (cosmic inflation, cyclic universes) propose mechanisms to reset entropy, but within the standard model, the universe is running down toward maximum entropy, reflecting the deep asymmetry between past and future.