A Sub-Boss is any Leader/Boss enemy NPC whose defeat is a requirement to unlock and/or further an event.
In other words, these Bosses are usually either the "right-hand men" of a Main Boss, are regional overlords in a section of Too Human (ex. the head of a gang of Common Bosses), or they are bosses whose importance is directly tied to their defeat which unlocks something in the game (ex. an event, a gate leading to the Main Boss in the region, etc.). This is contrasted by Main Bosses (who are key figures that move the storyline ahead), and by Common Bosses (who are more like Sub-Bosses, except that they are far more common and have no importance in the game beyond their existence at that point in time).