All things can suffer critical hits. Creatures that were formerly immune to critical hits now get a bonus to AC (as decided by the DM depending on the monster) against rolls to confirm criticals. For example, let’s say a Hammerer Golem (AC 24) has a +4 Resist Criticals score. If you roll a natural 20 against it, you then have to hit AC 28 on the roll to confirm to actually make it a critical.
This also means all monsters are sneak attackable (but get a bonus to AC against Sneak Attacks equal to their Resist Critical bonus) and flankable.