"Earthy" is broad. Generally speaking when it comes to cannabis, "earthy" to me means it smells like dirt, wood, even "oily" strains can be perceived as "earthy".
The absolute two best examples I can think of right now, are GMO and Peanut Butter Breath. Both are super dank, both definitely impart an "earthy" taste when smoking.
It's odd how "spoiled" things are so coveted! Foods and perfumes are prime examples; people love to eat fermented, spoiled shit throughout the World, and oud & ambergris are extremely popular perfume ingredients, ambergris is especially gross if you know what it is. Why should weed be any different? Our scene is littered with cheesy, dirty feet smelling strains that people love----which are also "earthy" if you ask me.