What is Heat current density?

Example 25.3 (Conductivities)

Metals (electrons carry the heat as well as the current — conductivities track each other): copper $400\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, aluminium $240\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, steel $50\,\mathrm{W}/\mathrm{m}/\mathrm{K}$. Insulating solids: concrete $1.5\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, glass $1\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, brick $0.8\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, wood $0.15\,\mathrm{W}/\mathrm{m}/\mathrm{K}$. Liquids: water $0.6\,\mathrm{W}/\mathrm{m}/\mathrm{K}$. Gases: air $0.026\,\mathrm{W}/\mathrm{m}/\mathrm{K}$ — and the best insulators, glass wool or foam at $0.04\,\mathrm{W}/\mathrm{m}/\mathrm{K}$, are mostly still air, trapped so that it cannot convect. Four orders of magnitude from copper to air.