The term served to distinguish those who were captains by rank from
Once an officer had been promoted to post-captain, his further promotion was strictly by seniority; if he could avoid death or disgrace, he would eventually become an admiral (even if only a yellow admiral).
In the Royal Navy of the time, an officer might have a rank, but not a command. Until the officer had a command, he was "on the beach" and on half-pay. An officer who was promoted from commander was a captain, but until he was given a command, he was on half-pay. Once the captain was given a command, his name was "posted" in the "Naval Gazette."
An officer "took post" or was "made post" when he was first commissioned to command a rated vessel — that is, a ship too important to be commanded by a mere commander. Unrated vessels could also in some cases be commanded by post-captains. Being "made post" is portrayed as the most crucial event in an officer's career in both Forester's Horatio Hornblower series and O'Brian's Aubrey-Maturin series.
After 1795, when they were first introduced on Royal Navy uniforms, the number and position of epaulettes distinguished between commanders and post-captains of various seniorities. A commander wore a single epaulette on the left shoulder. A post-captain with less than three years seniority wore a single epaulette on the right shoulder, and a post-captain with three or more years seniority wore an epaulette on each shoulder. (National Maritime Museum Uniform Collection, see below.) In the O'Brian series, Aubrey "wets the swab" -- that is, he celebrates his promotion to commander and the acquisition of his "swab" or epaulette with the consumption of copious amounts of alcohol.
Note that the term was descriptive only: no-one was ever styled "Post-Captain John Smith".