Arrow of time
Tldr
The arrow of time is the concept which posits the one-way direction, or so-called asymmetry of time. It was developed in 1927 by Arthur Eddington.
Most microscopic physical processes are believed to be mostly time-symmetric, meaning that if the direction of time were to reverse, the theoretical statements describing them would remain true.
At the macroscopic level, it often appears this is not the case → there is an obvious direction/flow of time.
Example
An apple falling from a tree (gravity) is considered to be T-symmetric, implying that recordings would look equally realistic forwards and backwards. However, the apple bouncing off the ground & slowly coming to a stop is not time-reversible: the kinetic energy of the apple is dissipated & entropy is increased.
Entropy is one of the few processes that is not time-reversible. According to the statistical notion of increasing entropy, the “arrow” of time is identified with a decrease of free energy.
According to Sean Carroll, the set of all arrows of time are a result of the relative proximity in time to the Big Bang (this sounds ludicrous given the consistency of entropy’s time-asymmetry, but it sounds cool).