A resin is a viscous fluid secreted by a tree. Using a resin wedge (which is made on an Anvil), a tree can be nicked, causing it to slowly yield a quantity of resin over time. Resin comes in various types, named after the type of tree that produced it.
I've noticed that at night trees harvested for wood will allow you to harvest resin. I have no Resin Wedge, yet I have harvested Resin in this manner. -Zoon
It may be that someone else nicked the tree before you got there. - Denu
It's actually -because- somebody else nicked the tree and is waiting for the resin to accumulate - kap0w