Terpenes are aromatic oils that are naturally produced in most plants. 
This is why nature smells good and fresh air feels good. 
Cannabis plants, both industrial hemp, and marijuana produce higher levels of terpenes than most other plants. This is why they are famously associated and different varieties have unique “personalities,” smells, and effects.  
The hops plant, used when brewing beer, also produces high concentrations of terpenes, in turn affecting how different beers smell, taste, and sometimes their effects too.
Like the cannabinoid profile, the terpene profile of a product can greatly determine “the effects” of cannabis products.