Is this stuff poisonous?  No. Rosin is not poisonous and is actually used in both making medicines and chewing gum. It is also used in another form in soft drinks.  Some people even use it for flavoring food.

Where does it come from?  Pine trees, Spruce trees, etc.  The resin is also called pitch. Frankincense is resin obtained from trees of the genus Bosu’elliaIt. It is soluble in alcohol which is good to know if you get pine sap on your clothes.

The Oxford English Dictionary defines resin as: 

A highly viscous, sticky, flammable, typically aromatic substance secreted by certain trees (esp. conifers, notably fir and pine) and other plants; (as a count noun) a particular substance of this kind; spec. such a substance which is insoluble in water (and so distinguished from a gum).
  

Resin commonly consists of terpenoid hydrocarbons, together with other organic compounds. Various forms of resin are harvested, often by making incisions into the bark of the tree, and used in varnishes, adhesives, medicines, and other products. Amber is a kind of fossilized resin.
  accaroid, copaiba, copal gum kauripine resin, etc.

Rosin is this substance in a solid state obtained as a residue after the distillation of oil of turpentine from crude turpentine.

The colour of the product (yellow, brown, or black) depends on the continuation of the heat employed.