Some warriors and creatures in the Warhammer world are almost completely fearless, or are such grizzled veterans that situations that would make lesser troops panic have no effect on them. This will be specified in the unit's entry.
Troops that are Immune to Psychology (sometimes also called Immune to all Psychology') automatically pass all their Panic, Fear and Terror tests and are not automatically broken if defeated in combat by outnumbering Fear-causing enemies, but take the Break test as normal.
Troops that are Immune to Psychology may never flee as a charge reaction - they are far too proud and brave (or too dim-witted) to do this!
These troops have to take Break tests as normal.