Player Heads were rare artifacts that were the heads of select 2b2t players.
The Player Heads were originally created when the Tyranny hacked into Hausemaster's OP alt, allowing them to do whatever they wanted. One of the things they did was spawn in the heads of many players. While it is unknown the original amount of heads spawned, the number drastically rose during the 11/11 Dupe.
Player Heads were usually sold by merchants for either real money or in game items (this option was usually difficult due to Gapples no longer being wanted by many on the mass market, but other items were commonly used instead).
Player heads were removed by Hausemaster at the same time as a bunch of other "illegal" items such as pig spawners, stacked armor, and enchanted glass, even though unlike all the other things he removed they didn't affect gameplay at all and were just rare, purely decorative items that formed a part of 2b2t's history. Hausemaster faced a lot of backlash over this decision, but player heads remain unobtainable.
For a long time, mob heads were unable to be obtained legitimately, due to Charged Creepers being unable to exist because of lightning being disabled. However, players still used backdoors, exploits and dupes to obtain them anyway. When lightning was re-enabled, it became possible once again to obtain mob heads legitimately.