Marks of Honor have seen some changes since they were first introduced during Legion, although they still serve much of the same purpose, to buy cosmetic transmog sets of old PvP gear.
Players can earn these marks by Winning PvP Battlegrounds, Arena's, and Skirmishes by opening the strongbox that is awarded.
Marks can also be a reward for certain PvP World Quests in the Shadowlands
Players can also trade in 2000 honor points for 5 Marks of Honor by talking to Purveyor Zo'kuul in Oribos.
WoW Shadowlands: How To Obtain Marks Of Honor And Where To Spend Them
Marks of Honor can be spent at each expansion's PvP vendor, with the only exception being Shadowlands, as the sets are still a part of the current game, and will be available likely when the next expansions laucnhes.
Marks of Honor can be exchanged for complete sets of gear (cosmetic only) based on the seasonal look when it was released.
Individual pieces will cost anywhere between 1-5 marks, heirlooms 8-10, and full sets costing 12 Marks of Honor.
Heirloom upgrades will cost 14 marks, mounts 15, and a complete weapon set from that expansion costing 80 Marks.
The Marks can be used for all seasonal armor sets dating all the way back to season one of the Burning Crusade, including all the weapons that could be purchased at that time.
For collectors, as of Legion, all armor ensembles will cost a total of 336 Marks of Honor for a single class.
Weapons are a lot more expensive, costing a total of 1760, so it is recommended to collect these on a Warrior as they have access to all weapon appearances.