The term microstate refers to the behavior of atoms and molecules in terms of how energy is redistributed among the particles in a system. A microstate is the arrangement of molecules within a macrostate, that is, within a system with very specific properties. If atoms have more energy and freedom of movement in a macrostate, there are more microstates possible.
In the coin analogy above, we can see while the microstate (arrangement of the pennies) changes, the overall characteristics of the macrostate (6 tails, 6 heads, all pennies, made of copper) all remain exactly the same.
The more microstates available to the system, the greater the entropy.