Threshold

[Sushidwarf]

but each horse and PC must make
an advanced TN 13 Dexterity (Climbing) test with a
success threshold of 9.

New GM here...
Can someone explain what they mean by threshold of 9...

[Admiral Yacob]

Check out Advanced Tests on page 18 of Set 1's Game Master's Guide. A threshold is part of an advanced test and is supposed to be the totals of all the dragon die rolls you get on the multiple attempts. I don't want to quote their published material so I don't have a better explanation, I'd recommend reading that section, it's about 1/2 a page.