Hysteroi Pezhetairoi are an improved unit of pikemen, having a greater degree of training and equipment compared to their predecessors.
Can Form Phalanx
Early in the 2nd century, after having fought the Romans in a few ruinous wars, the Makedonian army modified its pike phalanx accordingly. Gone were 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 Makedonia 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 Makedonian Wars, in which Makedonia was fighting a hopelessly outnumbered battle against the juggernaut of the Roman republic. They augmented their numbers by adding non-Makedones 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 Makedonia, whose population had been depleted to a large extent due to eastern migration of many citizens.