Hysteroi Pezhetairoi are an improved unit of pikemen, having a greater degree of training and equipment compared to their predecessors.
Early in the 2nd Century, after having fought the Romans in a few ruinous wars, the Macedonian army modified its pike phalanx accordingly. Gone was the single greave and fully bronze shield, not to mention the linothorax. They were replaced with mail armor and a wooden composite (plywood) shield of similar strength but far less weight. They were retrained to a great degree, and many Thracians and Illyrians who settled in Macedon were added to the rolls. These men have a greater degree of training and equipment compared to their predecessors, and are better able to stand up to the mobile and disciplined troops of Rome or the Greeks to their south.
Historically, the late pikemen were a development of the Macedonian Wars, in which Macedon was fighting a hopelessly outnumbered battle against the juggernaut of the Roman republic. They augmented their numbers by adding non-Macedonians to the rolls and trained those that they had far better, using new methods. The result was better than the earlier pikemen, but still not enough to defeat the Roman armies at the state of readiness they were at after the second Punic War. If Rome could defeat Carthage, which once had a massive amount of capital and excellent manpower to draw upon, then she could certainly defeat little Macedon, whose population had been depleted to a large extent due to eastern migration of many citizens.
A unit unique to the Makedonia faction, the Hysteroi Pezhetairoi are a heavier version of the standard Pezhetairoi. They wear stronger body armour and are better in a sword fight, thus being less vulnerable to flank or rear attacks. This means you can deploy them aggressively into the thick of combat, as long as you bear in mind that any phalanx, regardless of equipment or quality, must be supported by other troops. They are identical stat-wise to Pontic Chalkaspides.
Hysteroi Pezhetairoi become available after the "March of Time" event. Note that you can still recruit the lighter Pezhetairoi at the same time.
Compared to stock Pezhetairoi, they have a significantly wider area of recruitment, better armour, and better melee capabilities. The only downside of this unit is its decreased stamina compared to its predecessor unit. Thus, Hysteroi Pezhetairoi are a very important unit to a Makedonia player, and especially useful when fighting against the melee-focused factions of Western and Northern Europe.