The monstrous infantry category covers things like Trolls, Ogres and Minotaurs - creatures that are man- shaped and fight on foot, but are two or three times the size of normal infantrymen. We could perhaps have honestly called this category 'Big Infantry', but it does lack a certain gravitas. For the most part, monstrous infantry work exactly the same as normally(i.e. follow the standard rules) but have a couple of extra facets to represent just how massive they are.
Monstrous Infantry have thespecial rule.
As monstrous infantry are so much larger than normal troops, they require fewer warriors to fill out a rank. Where most troop types need five models for the rank to count towards rank bonus,and so on, a unit composed purely of monstrous infantry needs only three models. Essentially, wherever the rules say 'a rank of five or more models' treat it as reading 'a rank of three or more models'. Following the same logic, a unit of monstrous infantry only needs a frontage of six models to count as a horde, rather than the normal ten.
A monstrous infantry model can make as many supporting attacks as are on its profile, up to a maximum of three, rather than the usual one supporting attack.