We have weapons that have "rending" but we can't find out what rending does. Anyone know?
Rending is not a special quality of a weapon. It is the weapon's Damage Type as introduced on page 168 under Weapons.
Rending is the kind of damage done by swords, blades and claws. In contrast to for example explosive damage, done by grenades, energy damage done by laser weapons or impact damage by bullets or falling from height.
On page 256 under the heading Injury and Damage Types the core rulebook states:
The type of damage inflicted is usually only important for determining critical effects resulting from critical damage
So if you take critical damage from a weapon with the damage type of Rending, you consult the critical tables 8-19 through 8-22 on page 264 and 265 that say "rending critical effects".