microstate
Toss four coins and call out 'two heads' — that is what you see from across the room, the big picture. But 'two heads' can happen in six distinct ways: heads on coins 1 and 2, or 1 and 3, and so on. Each of those exact, coin-by-coin arrangements is a microstate. The fuzzy summary you announce ('two heads') is the macrostate.
In thermodynamics a microstate is one complete, fully specified arrangement of all the particles in a system — every molecule's position and speed, or every quantum of energy's exact home. A macrostate is the handful of bulk properties you can actually measure, like temperature, pressure, and volume. A single macrostate is almost always compatible with a staggering number of different microstates.
Why it matters: this is the hinge between the microscopic and the everyday. Entropy is essentially a count of how many microstates share the same macrostate — the more ways to arrange the parts, the higher the entropy and the more likely the macrostate. The honest caveat: the number of microstates is astronomically large (think 10 followed by trillions of zeros), so we never list them; we count them with statistics.
Spread four energy quanta among three atoms and there are 15 distinct ways to do it — 15 microstates — yet from outside the lump just has a single temperature. Double the quanta or the atoms and the count explodes, which is why warm, crowded systems have far more entropy than cold, sparse ones.
Many microstates, one macrostate — entropy counts the 'many.'
Do not confuse a microstate with a single particle's state. A microstate fixes the state of every particle at once; it is a snapshot of the whole system, not of one molecule.