Answer:
He meant that the huns were well-trained Cavalries which were high skilled in fighting from their horses. It seemed impossible to knock them down with weapons.
Step-by-step explanation:
The huns were a group of nomadic warriors that conducted series of attack to various territories owned by European empires. They had the reputation as capable horsemen and terrorize many Empires in Europe due to their fighting prowess.
They're originated from Asian continent, but historians can't really pinpoint the exact country where they're originated. (The most popular consensus is that they're originated from modern day Kazakhstan)