In physics, entropy refers to the amount of order in a given system. Irreversible processes, such as combustion increase entropy. This means that energy is more evenly distributed inside the system after the process. A log can burn but will not 'unburn' itself. The energy stored in the chemical bonds in the molecules composing the log is dissipated in the form of light and heat, entropy has increased. See any physics text for a more formal definition.
The QFS uses the term entropy in a special meaning. Entropy is used to mean fall into chaos, specially societally and individually. Entropy is the opposite of creation. Creation involves ordering things so as to create meaning. Increase of entropy or destruction destroys meaning and increases sameness. A societal collapse and the accompanying reverting from civilized into purely survival oriented behavior is an example of increased entropy in this sense.
In a metaphysical sense, subjectivity and wishful thinking represent entropy. This is so because the divide between the perception and reality is in itself a form of chaos and loss of information and cohesion. Service to self (STS) is associated with the concept of entropy in the sense that this seeks to limit the free will of others and deprive them of their energy and creative potential. By exploiting others, the STS entity may create a local peak of power and order or cohesion but this is at the expense of greater depletion of the potential of the exploited. Thus entropy increases in the system as a whole.