The term trash refers to units not requiring silver to produce; that is, pikemen, skirmishers, and light cavalry. While these are far from the strongest units available, they are cheap and "throwaway" units, units that can be produced in mass quantity early on, and are thus found in twelfth-century armies when making militia or archers can seriously hamper a player's advance.
However, there are civilisations and technologies in Age of Chivalry that can enable trash units to possess even greater strength than many a costlier unit. It is often a very good idea to have light cavalry along with the standard pike-and-skirmisher army, as onagers can very easily counter these - just another example of the increased importance of combined arms in Hegemony.
The term actually originates in Age of Empires II and is often seen as the base of early combat in that game. In Hegemony, it is very hard for a player to aim for a quick advance to the 14th century because the availabilty of rams and other powerful units allows for quicker assaults.