What do H, L and M mean in Frosthaven?
The three material resources: H is Hide, L is Lumber, M is Metal.
Materials come as Hide, Lumber and Metal, printed as H, L and M on loot cards, building costs and treasure entries. They are the resource you spend on buildings and crafting.
Material resources in a character's personal supply are for crafting items. Material resources in the Frosthaven supply are for building costs and cannot be used to craft.
The six herbs — Arrowvine, Axenut, Corpsecap, Flamefruit, and the rest — are a separate resource from the three materials and are used for potions.
“There are three material resources: L Lumber, M Metal, H Hide.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.